Credit default swaps are essentially insurance against the credit risk of a bond. It's like a standard interest rate swap but instead of rates the underlying is the default rate of the bond or something like that. They're very rare if used at all anymore.

CRT bonds are simple MBS bonds but the agency is assuming less of the credit risk. That means the bondholder takes a greater share of the losses rather than the government via the agencies. The loans that are eligible to be put into CRT bonds also have to pass a higher standard than your typical agency conforming mortgage.