Accidental is the term used for Sharps (#), Flats (b), and Naturals.
Accidentals are used to create pitches that are not in the key signature or to change other accidentals that happen earlier in the bar. Accidentals remain in effect for the duration of the bar. In other words until after the next bar line. What we usually see is the Natural the Sharp and the Flat, but there are times when the Double Flat and the Double Sharp are needed.
The Natural Sign
The Natural would remove any expected sharp or flat. So in this case, if you had a key signature that had a C sharp in it and you needed a C natural you would just place this natural sign in front of it and that would remove the sharp.
A Sharp raises the pitch by one half step. On the piano keyboard, a half step is the next key up or down from the note. So if we were on C and we wanted to go one half step up, C# would be the next note. If we were on C and we wanted to go one half step down, Cb would be the next note.
A Flat lowers the pitch by one half step. So if we start on C, lowering it by one half step would be Cb.
The double sharp raises the pitch by one whole step, so if we started on C and wanted to play C double sharp, go up two half steps. That would be a C double sharp.
A double flat lowers the pitch by one whole step.A wholestep is made from 2 half steps. So if we wanted to go one whole step down from C, we will go down 2 half steps.