JSONP is JSON with Padding. Browsers prevent Cross-site scripting of JSON and XML across different domains for security. This means you can only pull in XML or JSON from the same domain that it's being requested from. JSONP bypasses this security measure, so you can pull in data from other websites into your own with out the API provider making an exception for your site. If you look at the actual data, JSONP is exactly the same as JSON excpet it is wrapped it "(JSON);" (replace JSON with the actual JSON data).
For the sake of this class, a web service is server side code that spits out information from a database via a http:// page. In the instance we'll be looking at, the server always spits out the data in a format client side languages can understand, XML, JSON, or JSON P. More specific data can be retrieved from these pages by passing URL paramaters along with the URL itself.
People have been writing client-side code that pulls data from multiple APIs and does cool useful stuff with it, like the google maps / craiglist for-rent mashup: pad mapper. Twitter has a list of twitter mashups