Most LARPs can be classified as either skills or class based depending on how the game is structured. In a class-based system, you choose a character class (Fighter, Rogue, Cleric, Mage, etc.) which determines what abilities you have. As you gain experience you gain new abilities, but only those which are available to the class of your character (i.e. as a Rogue you could learn to pick locks, but you couldn't learn healing spells). In a skills-based system, you may choose a base character class (or not at all), but then as you gain experience you are able to choose what abilities you wish to "purchase" for your character from the pool of available skills, thereby customizing the character to your own tastes (i.e. as a Rogue, you might choose to learn a little bit of healing magic to help you out in a pinch, but it might "cost" you more to buy that ability than if you were a pure caster).
This site is a reference primarily for players of fantasy-based LARP systems such as NERO International, but players of other style LARPs may also find information of use.