Essentials is a hugely popular group of plugins, which has multiple jars, which serve different tasks.
Essentials - the main part of the plugin which handles signs, most of the commands, and the economy api.
Essentials Chat - handles chat formatting and prefixes
Essentials Spawn - handles spawning and respawning
Essentials Anti-build - building permissions system
Essentials Protect - a basic world protection system
Essentials Groupmanager - full permissions system (standalone)