What is the difference between MSP and Fabric CA?

MSPs are in charged for members certificates and authentication. MSP is pluggable, so the default Fabric MSP implementation uses PKI methods and X.509 certificates. You can use commercial CAs or Fabric CA to generate the keys and certificates needed to configure an MSP.

For more information read the Fabric CA documentation and https://hyperledger-fabric.readthedocs.io/en/latest/msp.html

MSP is a Membership Service Provider - pluggable interface to support variety of credentials architectures, basically offering abstraction layer for membership orchestration architecture. MSP abstraction provides:

1. Concrete identity format
2. User credential validation
3. User credential revocation
4. Signature generation and verification

While Fabric-CA used to generate certificates and keys to actually initialize MSP facilities. Fabric-CA is a default implementation of MSP interface to cover identity management.