EEL is invented by Cockos and often mistaken for this other language which is named the same but otherwise has no similarities (Eli posted a link to it).
Concerning Reaper EEL is used for 3 things:
- JSFX
- EEL ReaScripts
- Video processor
All 3 have their own dedicated set of functions but there are many things in common.
For example the stuff
here (JSFX basic code reference) is mostly available for video-processor too.
You'll find 3 kind of loops there too (for, while, do-while).