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 would be to get the visitor interested in the book by describing all its benefits and then have an easy way of purchasing the book. If you had a newsletter, your CTA would be to have them subscribe to the newsletter.
For a CTA to be effective, the action should stand out from the rest of the content on the page. It should create a desire in the visitor, not appear like a gimmick or trick, be bigger than other content on the site, and appear at the top and bottom of your content.