http://www.acm.org/sigs/sigplan/conferences/author-info/pughadvice.html

What is an Extended Abstract?

An extended abstract is not simply a long abstract. An extended abstract should contain references, comparisons to related work, proofs of key theorems and other details expected in a research paper but not in an abstract.

An extended abstract is a research paper whose ideas and significance can be understood in less than an hour. Writing an extended abstract can be more demanding than writing a research paper.

Some things that can be omitted from an extended abstract: future work, details of proofs or implementation that should seem plausible to reviewers, ramifications not relevant to the key ideas of the abstract.