Hyperledger Composer is a complement to Hyperledger Fabric. Composer allows you to model your roles, assets and transactions and it can generate some of the lower-level code for you. With Hyperledger Fabric, you would be writing this code yourself.
Hyperledger Composer cannot (yet) exploit all of the features of Hyperledger Fabric, but that is changing rapidly and the intention is not to have it place any limits.