Symbol["name"] refers to a symbol with the specified name.
All symbols, whether explicitly entered using Symbol or not, have head Symbol.
x_Symbol can be used as a pattern to represent any symbol.
The string "name" in Symbol["name"] must be an appropriate name for a symbol. It can contain any letters, letter-like forms, or digits, but cannot start with a digit.
Symbol["name"] creates a new symbol if none exists with the specified name.
A symbol such as x has a name "x".
If Symbol["name"] creates a new symbol, it does so in the context specified by $Context.
See The Mathematica Book: Section 2.3.4, Section 2.6.12 and Section A.1.4.
See also: SymbolName, ToExpression, Unique, Remove.