Squid vs caching products like memcached

What’s the difference between the reverse proxying features of Squid and a caching product like memcached?

I don’t necessarily mean specific comparisons of both products (e.g. performance), but rather explanations of what both types of products do. I understand that there are some large-scale websites out there that make use of both, so clearly they are better at different things and both have a place in a given architecture.

As a newbie, however, I’m unable to determine how they both come together and fit into an architecture to make a website more scalable, so your help would be really appreciated.

Thanks.