A call-to-action (CTA) prompts a user to do what you want them to do. It literally calls them to take an action. A CTA is a prompt on your website that leads a user down through your conversion funnel. The action is usually one that leads to a sale or a sign-up. Its goal is to evoke an immediate response (“Call now!” or “Buy now!”).
Most often, it takes the form of an actionable UI element, such as as button. It could also take the form of actionable information, like a phone number or other contact information. Split testing your call-to-action is important, and should be done often. Experiment with color, verbiage, and placement to see what works best.