More correctly, the fifty sounds refer only to the fifty basic characters of the Japanese syllabary. These sounds are the ones that do NOT contain the "nigori", or "han-nigori" sounds, i.e., the ones which are made by adding strokes to basic characters to form the hard sounds, e.g., hu -> pu -> bu.

These sounds are traditionally ordered in a poem, created by a buddhist monk named Kukai. I'm posting the poem and translation on the hiragana node.