In music, sharp means that a given tone should be one semitone higher than usual. Thus, C-sharp is one semitone above C. Sharp keys on a piano will be one key higher than the base note; C-sharp is the black key above C, while E-sharp is actually F.
In many cases, a sharp note will be the same tone as the flat note of the note above. For example, C-sharp is the same tone as D-flat. This equivalence is referred to as enharmonic. It is important to distinguish between them, however, since they will have different positions in the harmonic structure of the music.
In some rare cases, one will see a double sharp. This means that the tone must be raised by two semitones, or one full tone.
In musical notation, sharp is indicated by the symbol '♯', and double sharp by 'x'.