Nearly all action RPG's had linear skill systems where the progression of an active skill was preordained by the developers. If you were a fire user, you would progress from a weak fire spout to a more powerful fireball, and eventually to something big and flashy like a flame wave.
In Path of Exile, active skills (i.e., skills that you use on command) are itemized onto "Skill Gems". These gems confer an active skill to the player when they are socketed into equipment and gain experience as the player slays enemies.
A choice of staple skill gems are given out as quest rewards throughout the game to ensure that players have a variety of skills available. You’ll discover rare advanced skill gems out in the wilderness and dungeons. Like all items, these can be traded between players. Some of the more devastating skills are extremely valuable.
Gems can also level up. When a player is awarded experience, all gems in equipped items gain 10% of that value. This gain is in addition to the experience earned by the player. When a gem is removed or traded, it retains its experience. It’s possible to specialize in leveling up valuable gems and trading them to other players. We haven’t announced the number of levels that gems will have, but it takes a substantial amount of experience to reach the highest levels of gem potency.
Gems have attribute requirements based on their type and level. For example, a higher level cleave skill gem requires more strength than a low level one. If your character doesn’t meet the attribute requirements of the new level of the skill gem, then the experience value sits at the maximum of the previous level until you next gain additional experience and also meet its requirements.
When using equipment with linked sockets, Support gems can be linked to Skill gems to augment the skills effect. You can link multiple support gems to a given skill to create interesting and powerful combinations. The following is a list of the announced support gems, and their effect on the skill they are combined with.