Clear[, , ... ] clears values and definitions for the .
Clear["", "", ... ] clears values and definitions for all symbols whose names match any of the string patterns .
Clear does not clear attributes, messages, or defaults associated with symbols.
Clear["form"] allows metacharacters such as *, as specified in Section A.3.13.
Clear["context`*"] clears all symbols in a particular context.
Clear is HoldAll.
Clear does not affect symbols with the attribute Protected.
See The Mathematica Book: Section 1.2.2, Section 1.7.1, Section 2.4.3, Section 2.6.12 and Section A.5.4.
See also: Remove.