you can actually go to the chanel reference library to check the differences.
2.55 aka reissue is the classic flap bag in distressed crinkled leather/metallic/patent with new lock (without the CC logo) and new chain, comes in different sizes from 224 to 228. it is not a 'classic classic' unlike the classic flap, which is always around. reissue is seasonal and quite limited in quantities.
classic flap comes in caviar and lambskin, unlike the reissue. it has the traditional leather weaved through chain and has the CC logo lock. it comes in mini, small, medium/large (it's actually the same, some people call it med, some call it large), jumbo.
the similarities are that both have flaps, interior pockets inside (exactly the same) and come in gold/silver hw.
hope that helps.