Also known as "snap rings", retaining rings are small metal parts used to keep other components from sliding on a shaft, e.g. gears on an axis, or in a bore (hole).
Many names exist for the same types of retaining rings, and one name may be used for varying types - in an attempt to reduce confusion, we're including the DIN numbers and German names as found at Schraubenlexikon. And of course, photos and ideally a multi-photo imagemap are needed here!
|Synonyms:||C-clips, C-rings; DE: Sicherungsringe, Halteringe,|
Nutenringe, DIN 471/472 (external/internal)
|Suggested Tools:||circlip pliers|
Clips with little holes near the opening. There are special pliers for opening or installing them (one kind for the internal and another for the external variety as either squeezing or stretching force is required).
|Synonyms:||E-clips; DE: Sicherungsscheiben,|
"E-Sicherungsringe", DIN 6799
E-rings are external retaining rings with three thick sections, making them look like a bent letter "E".
Special installation and removal tools exist, but for the occasional teardown you should be fine with an appropriately sized flat screwdriver.
These have curved protruding ends that the appropriate pliers fit between. Only for external application. 
|Synonyms:||DE: Sprengringe, "Seegerringe", DIN 7993|
Yet another different shape of retaining ring, often called "circlips" but without the holes that aid installation.