Easy

Score multiplier: 0.5

"Reduces overall difficulty - larger circles, more forgiving HP drain, less accuracy required."

Osu

This mod is intended to lessen the difficulty of beatmaps, at a cost to your score. It increases circle size while 

Playing a song's Insane difficulty with Easy

Playing a song on insane difficulty without Easy

decreasing the approach rate(AR), overall difficulty as well as life drain; each of these by a huge amount. It also allows 3 "lives"; that is, when the HP bar is exhausted, the game pauses and it refills up to two times. This restart will stop the song temporary to fill up the HP bar and rhythm hint is the "ready" (depends on skin used). In addition, replays do not show the restart or restored HP, and so if a replay is watched where the player had to use a "life", it will appear as though the player continues to play at 0 HP.

However, some (if not most) argue that Easy Mode fails to make maps easier to play. They claim that circles become comically large, and AR becomes ridiculously slower. The mod has the potential of making the play area much more cluttered and harder to read, especially on insane or approved beatmaps. These points are elaborated upon in this feature request to improve the mod, by Lybydose.

Taiko

For Taiko mode, the Easy Mode decreases the slider velocity (which is, the same as reducing AR in osu! mode), overall difficulty and life drain. Note that the amount of hits which are required to finish a spinner is reduced due to the reduction of overall difficulty. Unlike Standard and Catch the Beat, the reduced note score requirement to gain HP in Taiko makes the HP bar fill up much more faster to replace the 2 revival chance as it is impossible to fail in the middle of the song.

Catch the Beat

CtB with Easy

CtB without Easy

In Catch the Beat mode, the effects are the same as in osu! mode - all fruits fall slower (same as reducing AR in osu! mode), the camera is zoomed in (increased circle size in osu! mode) and 3 lives are granted. Despite only using one axis, however, the fruits' larger size and slower falling rate causes them to clump together, creating the same alleged problems as in normal osu!.

Osu! mania

In mania mode, Easy mod decreses the life drain and overall difficulty.