Abstracts are concise summaries of your work. Typically, you submit an abstract first, then if accepted, develop a poster or podium presentation.
Follow the "call for abstracts" very carefully - especially word count and matching conference theme. If there are lots of submissions, reviewers may be looking for any reason to reject. Some calls ask for a "structured" abstract:
Additional tips:
Abstract Writing Checklist - Virginia Commonwealth University Libraries
Abstract Writing Tips - Virginia Commonwealth University Libraries