Homer is an epic Greek poem. Chapman's Homer is a different spin-off based on Homer.
Homer is a Greek poet, author of such works as the Odyssey or Iliad. Chapman's Homer is the translation of Homer's poems.
Homer is a yellow man, from the Simpson's. Thus Chapman's Homer is just a name for one of Chapman's friends who is similar to Homer.
Homer is a sponge. Thus Chapman's Homer is what Chapman uses to clean his dishes.