Call to actions or CTAs are a specific type of button which encourages the user to take a specific step. This is usually to buy something or register. They should stand out from the background, making them the next logic step for the user to take. The call to action is often paired with an explanatory and encouraging text.
Generally, the copy for a call to action should be written so it could be the end of the sentence “I want to …” i.e. “I want to register now”, “save 30%”