Seth Godin (The Big Red Fez) calls them bananas and your users are the monkeys. The objective is for the monkey to find the banana in less than 3 seconds (before they give up and leave).

“Force yourself to design each and every page with one and only one primary objective. That’s the banana. Make it big. Make it blue (or red). Make it obvious.”

» leemunroe.com/web-design-trends-call-to-action-signup-download-buttons/
» via twitter.com/Timvansas