A pub crawl means that you (and a group) are going to several (at least 4, possibly more) bars in a single night. You start at the first bar, you drink, then you all head over to the next bar. It is a planned activity. It's different from going to a single place for a night, or even going somewhere and deciding while you're there that you would rather go to a different bar.

Usually a pub crawl involves stopping at several pubs, spending some time at each one. So you are "crawling" from one pub to the next :)