The flat symbol alters the pitch of the note to which it is attached as well as any subsequent occurence of the same note (identical line or space) in the same measure. Notes with the same pitch name, but a higher or lower octave, are not effected. Any note with a flat that also has a tie across a barline carries the flat to the note on the other side of the barline. Notes in the new measure that are not tied to altered notes from the previous measure revert to their original pitch and are performed using the current key signature.
Also [Eng.] Flat; [Fr.] bémol; [Ger.] Be; [It.] bemolle; [Sp.] bemol.