Did Satoshi Nakamoto (in his blogs or the whitepaper) forecast that mining pools would exist?
Nakamoto did discuss the idea when others brought up the notion of cooperative mining, but I don't know if he wrote about it before anyone else came up with the idea.
You can search an archive of Nakamoto's communications
https://satoshi.nakamotoinstitute.org/posts/bitcointalk/threads/250/?view=satoshi

ribuck's description is spot on.
Pool operators can modify their getwork to take one additional parameter, the address to send your share to.
The easy way for the pool operator would be to wait until the next block is found and divy it up proportionally as:
user's near-hits/total near-hits from everyone