The sounds with the symbol of alteration are called altered sounds (sharped or flatted) compared with the natural sound they represent. As follows, you find alteration signs:
sharp  |
alters the note of a semitone to above; |
double sharp x  |
alters the note of two semitones to above; |
flat  |
alters the note of a semitone to below; |
double flat  |
alters the note of two semitones to below; |
Alterations can be: temporary, precautionary and constant.