What is an extended abstract?
An abstract is a preliminary submission that summarizes the contribution of a paper. There are usually strict limits on the length of an abstract, either in terms of words or of total characters (rarely do they exceed 1000 words or 5000 characters; they are often substantially shorter than this.

An extended abstract and a full paper are nearly the same; the primary difference is that an extended abstract tends to be somewhat shorter than a full paper; I've seen extended abstracts from 2 pages up to 6 pages, while conference papers run from 4 up to about 12 or 15, depending on the space allotted.

One other important difference—outside of computer science, extended abstracts almost never go through a formal peer-review process before being published, while a conference paper will typically have at least one reviewer. (I think this is the case for almost all such papers, but there may be exceptions.)