In Path of Exile there are three types of skills that are placeable. These are totems, traps, and mines. Each of these skill types allows the player to place a skill at a location and walk away, and have the skill activate at that location. Each type behaves a little differently, as described below.
Totems are objects the player can summon that either grant an aura to friendly targets near them, or cast a skill at nearby enemies. Totems can be seen by enemies, and can be destroyed by being directly attacked.
Traps can be thrown by the player and are triggered when an enemy approaches. When a trap is triggered it can target the single enemy that triggered it, or it can cause an area of effect such as setting the ground on fire and burning all nearby enemies. Traps are invisible to enemies, but can be destroyed by area of effect damage.
Mines are similar to traps, but they are placed at the player's feet rather than being thrown to a location. Instead of triggering when an enemy approaches, mines are manually detonated by the player at an opportune moment. Mines are also invisible to enemies, but like traps can be destroyed by area of effect damage.
In addition to specific totem, trap, and mine skills, there are also support gems for each type. The support gem will turn any compatible skill into a totem, trap, or mine depending on which support was used. For example, you could connect a Fireball gem to a Totem support and create a totem that shoots fireballs at nearby enemies.