McMMO is a plugin that adds a ton of mmo-esque game mechanics. This includes everything from player skills to abilities for tools.
Note: You do not gain xp from killing/taming mobs.

/stats -check your stats and skills
/(skill) -check out a mcmmo skill
/credits -check your credit count
/redeem (skill) (amount) -add credits to a skill

McMMO credits
McMMO credits can be purchased from the store or obtained via vouchers in-game.
Note: You cannot redeem credits to a child skill such as salvage.

Useful Links
Cosmic's McMMO guide can be found here.

The McMMO wiki can be found here.