An accidental is a sign written before a note which raises or lowers it's pitch. The new pitch is held till the end of the bar unless written otherwise.
Accidentals may be bracketed as a reminder.
A sharp raises the pitch of a note by a semitone.
Figure 1: Two sharp notes (G#, D#).
A flat lowers the pitch of a note by a semitone.
Figure 2: Two flat notes (Gb, Db).
A natural strips the note of any accidental alteration and restores it to its natural pitch.
Figure 3: Two natural notes (G, D).
A double-sharp raises the pitch of a note a tone, double that of a regular sharp.
Figure 4: Two double-sharp notes (G##, D##).
A double-flat lowers the pitch of a note a tone, double that of a regular flat.
Figure 5: Two double-flat notes (Gbb, Dbb).
Also see Scales | Intervals.