Here’s a pretty good, simple article that explains pagerank. It also has a chart that shows how many links you need from certain pages of certain pr’s in order for your page to have a certain pr. It’s pretty good.
It doesn’t explain however, how PR affects your search engine ranking. I’m not totally sure about this (nobody really is), but it is believed that your search engine ranking is determined by multiplying your pr times the rating your page recieved for the search terms the searcher entered.