So your buying some new forks for your mountain bike. Remember, don't buy them for the sake of it: cheap forks (such as Zoom) will probably not even work!, forks with too much travel will make your bike not handle right, and the wrong size steerer tube will not fit.
Always look for a creditable make (look for: Fox Forks, Marzocchi, Manitou and Rock Shox) when buying some new forks and always bare in mind what you will be using them for. Buying a XC fork for full-on downhilling is the worst thing you can do and the same goes visa-versa. Downhill forks have sometimes 200mm+ travel and try riding up a steep hill bouncing along on that! Remember with triple clamp forks, you cannot spin them all the way around so these are not ideal for dirt jumping.
Forks come in all shapes and sizes. First of all, you need the right steerer tube size: you can choose from 1 1/8th and 1.5 inchs. 1 1/8th is the most common but manufacturers are now starting to use 1.5 tubes on their freeriding range (trek, iron horse etc.)
Secondly, you need to make sure you have the right amount of travel for what you are going to use your fork for. Here is a rough guide for how much travel you should be using:
XC: 80mm-130mm, Dirt Jumping: 100mm-130mm, Trials: Rigid, Downhill / Freeride: 150mm-200mm+
Thanks for reading my guide and I hope it was useful!