Ah, GURPS! my favourite system. As an overview:
GURPS Stands for Generic Universal Roleplaying System, and that's an accurate description. GURPS can be used for just about any game, but it takes some work to set up. The basics of GURPS are simple: you have a bunch of character points, probably somewhere from 100 (Extraordinary human, baseline advnturer) to 1000 (Godlike, High tier superhero). You can spend these on attributes, skills, and advantages, and you can get more by taking disadvantages up to a limit, usually around 50 points or so. Skills are based off of attributes, and cost progressively more CP per level, up to a cap, while attributes and advantages usually have a static cost per level, if they come in levels. There are rules for technological skills, martial arts, and MANY different magic systems. These things mostly work well together, though there are various traps to watch out for. GURPS is based around realistic games, either set in the present, or in a mundane historical setting. It can do more exotic settings quite well, but it brings it's realistic approach with it. You can do ultra silly super cinematic games with it, but it takes a lot of work. I personally rather like this realistic approach, but if that's not what you want you may wish to use a different system unless you are familiar with GURPS.
Next, things GURPS has to offer:
- Detailed and varied character creation. You are not restricted to using classes, you can make any kind of character you can imagine (And have the points for).
- Detailed Combat: target and cripple individual limbs, civision between armor and dodging/blocking/parrying, detailed combat options.
- Realistic chracters: A powerful wizard does not automatically have the potential to shrug off a baseball bat to the head unless he casts a spell or spends character points on the ability.
- Many different magic systems: Default magic as skills, The power of Sorcery, Divine Favor, Psi, Chi, Ritual Path Magic, etc, etc.
- More options than you can shake a stick at: Want to do X? There are probably rules for it already.
- Less randomness: The likelyhood that you will randomly fail at something for no reason is significantly decreased.
Now, a quick guide on using GURPS. GURPS has dozens of optional rules for use in different situations, depending on the genre and how cinematic/realistic/gritty your game is. You'll need to decide before hand which of these you wish to use, and plan accordingly. You will also need to make sure you apportion enough points for people to make their characters, but not so many they get out of hand. You'll need to watch what disadvantages people take, and make sure they work within your setting and that people actually suffer disadvantages from them, and don't just ignore them. Finally, remember that GURPS relies very heavily on the active use of rule zero: The GM Makes the Rules. Thus, it's up to him to catch when someone is pulling a fast one and say "No, you can't put every single point in strength."