What do the icons in the Eclipse debugger mean?
What do the icon decorators in Eclipse mean?
What do the icons in Eclipse's Package Explorer mean?
What do the little letters on top of Eclipse icons mean?
What's the difference between the two error icons in Eclipse (the x in the red circle and the one with the light bulb)?
I just came to SO looking for this, didn't find it, and found it on my own elsewhere. But I thought it would be good for SO to have the answer for future reference; I wondered about them all the time when I was new to Eclipse (this was before I discovered SO).
This is a fairly comprehensive list from the Eclipse documentation. If anyone knows of another list — maybe with more details, or just the most common icons — feel free to add it.
2019-06: JDT Icons
2019-03: JDT Icons
2018-12: JDT Icons
2018-09: JDT Icons
Photon: JDT Icons
Oxygen: JDT Icons
Neon: JDT Icons
Mars: JDT Icons
Luna: JDT Icons
Kepler: JDT Icons
Juno: JDT Icons
Helios: JDT Icons
There are also some CDT icons at the bottom of this help page.
If you're a Subversion user, the icons you're looking for may actually belong to Subclipse; see this excellent answer for more on those.