http://web2py.com/books/default/chapter/29/4?search=cache#cache
When it takes a while to figure something out, like a complex query, you typically want to hold on to the result for a little while so that the next few requests don't have to do through the same process every time. It is used to increase performance. memcached is an external process that web2py can use to do caching. In large deployments, memcached has a performance increase over the standard web2py cache methods.
