ToLowerCase[string] yields a string in which all letters have been converted to lower case.
ToLowerCase handles both ordinary and special characters.
Variant upper-case characters such as \[CurlyCapitalUpsilon] are converted to their non-variant lower-case forms.
See The Mathematica Book: Section 2.7.4.
See also: LowerCaseQ, ToUpperCase, StringReplace, IgnoreCase.