Lots of registers, comparators, and a wide mux. Like Tricky said, they do not synthesis well in FPGAs so they are fairly rare to see these days in an FPGA. If you google around the description of a CAM should pretty much describe how you would implement one since they are fairly trivial (except if you want one that operates quickly).
Alternatively you can implement a latent CAM where you don't search the entire structure in parallel and instead use small memories that you search serially. That somewhat defeats the purpose of a CAM in some systems but maybe it's a small price to pay for what you are attempting to use a CAM for.
