Short for call to action, when referring to a website, CTA is an action that draws the visitors attention and hopefully leads to an action. For example, if you were selling an e-book, a call to action gets visitors interested in the book by describing its benefits, and providing an easy purchase method. If you had a newsletter, your CTA would be to have them subscribe to the newsletter.
For a CTA to be compelling, the action should stand out from the rest of the content on the page. It should appear at the top and bottom of your site, and displayed more prominently than other content. The CTA should create a desire in the visitor, not appear like a gimmick or trick.