In the unlikely event that you are not familiar with the term this refers to the control technique that is used in nearly every first person game on the PC since the mid 1990's. For reference I will outline the main details at the bottom of this post but many gamers will be intimately familiar with this already.
Mouselook has been a great boon to PC gamers. On the one hand its standardisation allows gamers to move easily from one first person game to the next with a very short learning curve. In the same way that learning to drive allows you to drive any car, learning mouselook allows you to play any PC first person game. Of course mouselook wasn't adopted as the standard by accident. It happens to be a suprememly fluid and intuitive control scheme for moving your character around in a virtual 3D world. In fact I don't believe a better scheme exists. Perhaps the clearest indication of this is the fact that console first person games which cannot implement mouselook due to the lack of a mouse invariably have some kind of auto-aim feature to compensate for the imprecision of the joystick equivalent.
Although Quake from 1996 is the game which popularised mouselook on the PC there were earlier games that used this technique. Wikipedia credits the Macintosh game Marathon from 1994 with the first implementation. I myself first came across mouse and keyboard controls when playing the 1995 game Descent. Descent is a flying game and the developers strongly reccommended the use of a joystick. The controls were configurable however and a post on usenet alerted me to the ability to set up mouse and keyboard control. I have never looked back.
In a fit of nostaligia I tried to find that usenet post using Google groups. I think it was probably this thread dated MArch 1995. I cant find Rob Markovic's original post but the link quotes his description of a mouselook control scheme. In modern implementations the WASD keys are generally used instead of the arrow keys that Rob recommends because they are better positioned for the left hand. I don't know what became of Rob Markovic but I think he is one of the great unsung pioneers of PC gaming. Thank you Rob.
By the way, Parallax the company who developed Descent gave birth to Volition who developed Freespace so I guess it is entirely fitting that I should again ignore Freespace's default control scheme to rig up mouselook
To be fair I should point out that mouse and keyboard control gives some people quite severe nauseous headaches. The ability to rapidly pan a 3D view around probably sends some confusing messages to the brain. I used to suffer from headaches like that myself but since the advent of fast graphics cards, high resolutions graphics and fast screen refresh rates I have not experienced this to any great extent. A couple of years ago I re-installed original DOOM and found that its low res graphics still gave me feelings of nausea after a prolonged playing session.
Perhaps the greatest problem with mouselook control is that it may soon cease to be. The hard economics of the gaming industry means that consoles are now the platform of choice for major releases. MMORPGs and Strategy games are fighting a valiant rearguard battle for the PC but all the major first person shooters now come out on console first.
Brief Description of Mouse and keyboard control (aka mouselook aka WASD) as used in PC based first person shooter games.
In its canonical form it goes like this:
Mouse adjusts the direction a person is looking.
W and S move the player forwards and backwards respectively. This feels very natural. The player walks forwards or backwards in the direction they are looking.
A and D slide the person sideways perpendicular to the direction they are looking. This is an unusual movement in real life (a kind of sidestep) but it became embedded in the standard due to the technique of circle strafing
Left Mouse click fires primary weapon at the item currently in the centre of the screen (often indicated by an aiming reticule).
Mouse wheel and or number keys change between weapons
The above points are almost universally adhered to but there are a few other conventions that are very common:
Space Bar: jump
Shift: Run or Walk
Q,E: Peek around corners
Right mouse button: alternate weapon or alternate fire mode or weapon zoom mode.