Shielder

Tasks: Block | Attack, only when it is safe | Defend other players
Equipment: 2 shields | One Handed Weapon
Description: The role of the Shielder is to hold the line. These players will be used in tactical positions to draw the enemies fire and defend positions that make the team vulnerable to attack. Shield walls allow the team to concentrate their attacks on a group of enemies at the same time, eliminating them quickly and without many casualties. Typically it is best to give the best shield to the person with the highest shield skill, so the shield will get the most usage. The best class for this is infantry, but other classes are good secondary blockers.

Attacker (sometimes called a Slasher)

Tasks: Attack enemy
Equipment: Two Handed Sword/Axe/Polearm
Description: The role of the Attacker is to deal as much damage to the enemy as possible. Usually the player will position themselves behind a group of Shielders to avoid taking damage. Attackers will be required to move between key positions on the map in order to support other groups of players. They're also called slashers because their long 2H weapons allow them to safely slash the enemy. As a note, the overhead slash attack work well in hitting over the shoulder of the shielder. Infantry and pikemen classes can both be good at this.

Skirmisher

Tasks: Attack enemy
Equipment: Polearms/Throwing
Description: The role of the Skirmisher is very similar to the Attacker. It is to deal as much damage to the enemy as possible, on top of what the attackers do. Usually the player will position themselves behind a group of Shielders to avoid taking damage and also behind the Attackers, as Skirmishers weapons allow them to strike from afar. A Skirmishers main job is to soften up the enemy from afar and then add additional slashing support ontop of the Attackers. Skirmishers will also be required to move between key positions on the map in order to support other groups of players. The only class that can really do this is pikemen.


Archer

Tasks: Attack and suppress the enemy
Equipment: Any bow and arrows
Description: The role of the Archer is to eliminate as many enemies as possible before they reach the player defences. With their fast fire rate Archers will be responsible for preventing the enemy from advancing for as long as possible. Once the enemy gets close they will retreat to a safe location and let the Shielders/Attackers deal with the rest of the enemy. Archers will also be key in taking out enemy archers. Archer is the class suited for this.

Sharpshooter

Tasks: Attack key enemy targets | Suppress the enemy
Equipment: Any crossbow and bolts
Description: The role of the Sharpshooter is to concentrate their fire on enemies that pose the biggest threat to the players. Due to the high damage and long distance shots a Sharpshooter can deal, they will be responsible for taking out heavily armoured enemies early on in the wave. They will also assist the Archers in eliminating enemy archers. Higher class archers are good at this, crossbows are great too because they have higher accuracy at lower levels.

Medic

Tasks: Heal players
Equipment: One or Two Handed Weapons| 2 x Medic Box + 1 Surgeon's Kit/Resurrection Tome
Description: The role of the Medic is to keep the team alive. Ideally the team will consist of at least 2 Medics which will allow them to alternate heals during and between waves, so that the team is well healed. A Medic is not required to attack the enemy (although they should be able to defend themselves in necessary). Any class can take this profession, if you find yourself in servers where there are many players who are much stronger and thus together you make it to later waves, you might be too weak to actually defend or attack. You can still be useful to the team if you focus on healing others at key points.

Engineer

Tasks: Construct defenses in strategic positions across the map
Equipment: Any weapon | Barricade or Heavy Barricade
Description: The role of the Engineer is to construct defenses in positions specified by the captain during the beginning of the game and then after every re-spawn. Ideally, the player will re-spawn at every possible occasion to bring more defenses. This is another great profession to take alongside your class, the deployable shields are useful and the barriers you can place later on become indispensable.

You have to launch Nord Invasion FIRST before you can register your character and start to level them up and buy things. When you enter Mount and Blade and select the mod Nord Invasion the game will launch. At the opening menu screen you select play Nord Invasion. Then you create a character, keeping in mind what the character's name is. When you join a server is will give you an activation code. This you will need to put into the registration page on the Nord Invasion website. If the message went by faster than you could capture it, you can push "L" to bring up game messages and you can even screen shot it if you need to but you will only need to input it once (default screen shot for steam is F12).

Normally this kind of collaboration is encouraged. However, In Nord Invasion, the next wave of enemies comes when the last enemy dies, so players like to "save" the last enemy by having one person take the initiative to block all their attacks so everyone has a chance to prepare for the next wave of Nords. Now that there is a timer between each waves, players don't use this method as widely but they might want to save closer to the final wave. Keep an eye out for when people say "save" or "save last" or if a player is blocking an enemy with their weapon and no other enemies are around , they're probably not in danger and are just allowing time for everyone to get ready for the next wave. People also tend to use their weapon to block, saving their shield for harder enemy waves. When players are in real danger of dying, they usually block with their shield.  (See the section on commonly used terms below)

The timer on the left currently does nothing but count down from five minutes. The timer on the right tells you how much time you have left to heal, repair your shield and reload. During the countdown the healing, repairing and reloading rates are faster so using the time between waves to prepare is even more effective than before.

It used to make sense to take multiple classes, but there are now hero classes after the 4th tier of your class and you only have a limited number of stat points, getting one after each level, so it is better to focus on one class exclusively with Medic or Engineer on the side.

There are 20 Waves total. A wave of Nords comes and when the last is killed there is a rest period that has a timer which counts down.  The waves follow this pattern:
1) Mixed wave of  Nords.
2) Challenge wave, meaning Nords of 1 type generally of higher ranking.
3) Mixed wave of Nords with a few heavier units than the previous mixed wave.
4) Boss wave/Respawn Wave.
Upon completion of that 4th wave, all the players that have died respawn. A wave indicator has been added to the game that tells you which wave everyone is on and how many to go before the players respawn.

Any character that is new is called a red shirt because they all start out with basically nothing... and a red shirt

Players will say this when they want you wait to kill the last Nord in a wave, which allows everyone to heal, refill their ammo, etc. They'll let you know when they're ready by either saying "R" or "Ready" or "Kill". Now there is rest period after each wave that allows you time to heal, repair, reload and fix up the base, so you may only see players calling for this in later waves when you get closer to the last wave with the Prince.

A role that focuses on attacking enemies, see classes and roles above.

A role that holds the font line of the incoming Nords away from the rest of the players, see classes and role above.

Stands for Nord Invasion, mod of Mount and Blade that you are playing.

Since the number of objects on the ground is limited, things that have been dropped earlier disappear as more Nords are killed and they drop their weapons. It is no longer necessary to juggle support items such as: Barricades, Medkits, Ammo boxes or Tomes. However, weapons can despawn and so players pick them up using "F" and then drop them again using "G", this "juggles" the item and prevents it from depawning.

Tome refers to a support item called the Resurrection Tome. It can be used to bring back all the players that have died and is usually used when the top 3 players are dead. Anyone can pick up and use a tome, however, to bring one in as a part of your equipment requires the highest tier of the Medic support class and each tome can only be brought in twice per map.

Stands for team speak, many players use this voice chat software to communicate in game, a lot of houses use it too. You can find it here: http://www.teamspeak3.com/

Since I've compiled some things from others posts, I wanted to thank the Nord Invasion community in general but here are some specific people that either posted helpful information which made it here or players in-game that I've found to be particularly helpful to new players.

MrGuarnere - Information on roles and classes
Renji - Supporting new players
Sosruko - Supporting new players
Fire and Blood - Supporting new players
Suns - Supporting new players
Sosruko - Supporting new players
Sargent Q - Edits to this topic
Tarrod - Edits to this topic
Slayer - Information on waves
Winter - Lots of support in the forum!