What do the icons in Eclipse mean?

• 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).