Player models are visual representations of a person or robot playing the game that they are placed in. Generally these are 2 legged, 2 armed, more or less human styled items, although as you can observe from the site, this is by no means a hard rule.
The player model for just about all games will usually consist of a actual 3d model file that you place in a specific directory within your installation of the game to use. Some of the player models for certain games will also require 2d art or 'skins' to go with the model as well to work properly. Additional items might also be available such as custom weapons and sounds. As a rule, anything that the author has included with the model or approved as such, will be in the initial zipped file that you will download. You can also expect to see a read me file in each that should give you some general instructions on where to place the model within your installation properly. If you have any problems with a player model, you should first read this file to make sure you have followed the correct installation procedure as well as any special requirements it may have.
If a game allows for user created player models to be used, it should have a way to select the player models you install on your own. Inspect your user manuals for each game's requirements, but it should be under something along the lines of 'Multiplayer' or 'Player Setup.' If a model does not show up were you expect, please check the individual FAQ's for each game as each has specific do's and dont's that are could trip you up.
"What is a Player Model?"
As before, a player model is 3d representation of other players in the game, whether they are other live gamers or robots. The model is actually just a visual placeholder for you to shoot at. The actually entity that a player uses in the game is called the 'bounding box.' This is a 3d volume that the game knows exactly what size and where it is at all times. It is also invisible, which is why we need to use the player models to see each other. The bounding box is what the game sees as you run around the levels. It is what collides with the walls and floors to keep you within the world. It is where the the rockets and bullets come from that you spray at the other players. It is also what is used to determine if you got hit in the game and how badly. Now a bunch of refrigerators running around the game is not real exciting, which is why the box is invisible and we use player models.
The player models are located in the game in the exact same spot as the bounding box. As you run around the levels, the game displays the 3d model doing different animations as per what actions you tell it you are doing. If you are running along, the model is displaying the run animations while it is pushing the bounding box around the level. If you jump the box will lift off the ground the prescribed amount and the model will display a jumping motion at the same time. So when you shoot the big bad guy, his own bounding box is there as well and registered that you have just hit him in the upper section of the box and with enough damage to kill him. The game then knows that it has to then display the model in a dieing animation and a proper amount of 'blood and bits', which you see in the game.
Basically the concept of player models is that it is visual trick. The game doesn't care what it is displaying, it merely is keeping track of the bounding box and displaying the proper visuals because it is told to do so. The game would work the exact same way with a bunch of cubes sliding around in the levels, but it wouldn't be as fun as shooting a large scary demon robot from outer space.
"I think a model is unfair or cheating. What can I do to stop it?"
An immediate concern that comes to peoples minds when the notion of using 3rd party models is that someone could make a model that's unfair, ie: it's to small to see, or below the floor (making it virtually invisible). This problem is easily remedied. If there are any models that you feel are unfair, you have the choice of simply not installing them or deleting them. For instance: You're playing along, racking up the kills, and this someone comes around the corner using this new model you just got. You shoot at his head, but the shot goes right through him and he puts your insides all over the wall. You can simply go in your game directory that holds the model and either delete the directory of that model or rename to something like model_unfair or something. Now anyone using that model will show up as the default character for that game.
"Whenever I see a certain model or a group of a lot of user created models, my machine lags out, why?"
It is possible that if you join a game and there are so many people using different player models that your computer starts running out of memory and frame rate suffers. If this becomes a problem, you'll have to pick your favorite models and either rename or delete the others.
Some models by themselves require a lot of processing power to to display, because of their polygon count or a variety of reasons. check the review or info page for any model you suspect, if a model is blatantly using a lot of power we will usually call it out in the discussion of the model.