Real time web refers to a product or service that works by delivering some kind of output from an input which has just happened. That time frame could reasonably be anywhere from milliseconds to a few minutes.

It almost always means page updates with Javascript, like AJAX but affecting output instead of input.

Twitter search (search.twitter.com) is seen as a good example of a real time web service, as their search results are tweets that have just happened. In contrast, the Google search index can be pretty old (e.g. http://twitter.com/bing/status/2559359562) and isn't updated fast enough to be considered real time.