What are the different forms of alliteration?

Alliteration is a poetic device that appears as repeated sounds, at stressed syllables. The Word alliteration itself is alliteration; it is marked by the double "L" sound at the stressed syllable. While it contains also a repeated "A" sound, the A's are at unstressed syllables, therefore non-alliterated.

There are three sub-forms of alliteration: assonance, consonance, and sibilance.

Assonance:

The repetition of vowel sounds within a line of verse for the purpose of supporting or continuing internal rhyme.

Consonance:

The repetition of consonant sounds within a line of verse for the purpose of supporting or continuing half or slant rhyme.

Sibilance:

The repetition of sibilants or "ss" sounds in a line of verse. Sibilance is more accurately a special form of consonance.

All assonance, consonance, and sibilance are alliteration, but not all alliteration is assonance, consonance, or sibliance.