An ISP, is a Internet Service Provider. Usually, they offer not just webhosting but for example also Dialup and DSL, dedicated lines, consulting, you name it.
An example:
http://www.exodus.com
Someone who describes himself as a webhost, just does the website related stuff. Shared, virtual and dedicated hosting.
Then again, a lot of smaller companies also call themselves ISPs. It's a mix, I guess. I just got the feeling that I am not of any help here.
Sometimes, both terms describe the same thing. I just like to distinguish them like that. Maybe one of the experts here knows better.
