Creepers within a 6 block radius of a player with a Defuse armour piece will become defused. This means the creeper will not be able to explode. Once a creeper is defused, it will remain defused forever.
Compatibility
If Bat Grenades is installed, bats will also become defused when within a 6 block radius of players.
Technical Details
Technically the creeper can still explode, if a player stands near it for about 27 minutes, but when it does explode, it will not do any damage to blocks or entities. If a defused creeper is ignited with a flint and steel, it will not move or chase the player and will explode after the 27 minutes noted above.