When the Init, Invoke or Query function of a chaincode is called, the fabric passes the stub *shim.ChaincodeStub parameter. This stub can be used to call APIs to access to the ledger services, transaction context, or to invoke other chaincodes.
The current APIs are defined in the shim package, generated by godoc. However, it includes functions from chaincode.pb.go such as func (*Column) XXX_OneofFuncs that are not intended as public API. The best is to look at the function definitions in chaincode.go and chaincode samples for usage.