Also called "voiced sound mark".
It is a diacritic placed beside certain kana to change their pronunciation. It can be used with hiragana or katakana.
Without dakuten: ka ki ku ke ko sa shi su se so
With dakuten: ga gi gu ge go za ji* zu* ze zo
Without dakuten: ta chi tsu te to ha hi fu he ho u
With dakuten: da ji* zu* de do ba bi bu be bo vu
* Note that there are two ways to write "ji" and two ways to write "zu". The more common are:
ji = shi with dakuten
zu = su with dakuten
The dakuten looks like two small strokes to the upper right of the kana.
Compare handakuten.