In Rift, portals, or rifts, to other planes periodically open in the wilderness. When a rift fully forms, creatures from beyond come through it and the ground surrounding the rift is terraformed to resemble the plane on the other side. Rifts will periodically spawn invaders who plow through everything in their path (including invaders from planes other than their own) until they reach their target player faction settlement. These invasions may conquer settlements and disable all services and quests there until the invasion forces have been defeated.
Rifts can be closed by killing their guardians. The "event" at a rift is divided into scripted phases, which if beaten quickly enough allow for "bonus phases" with more enemies to kill. When the final phase (often a boss of some kind) is beaten, rewards are distributed based on contribution (this was never balanced in beta) and the amount of total points accumulated in all cleared phases.
Sometimes zone wide events happen, spawning dozens of Rifts and potentially an end boss invader after enough rifts have been closed.
It's not as awesome as it sounds, though it's more interesting than the Garlean raids we get in FFXIV.
