Call to action buttons are those elements that demand an action from the user to activate/start/display something. We have several kinds of call to action buttons, from ‘download here’ to ‘sign up here’ and ‘buy it here’ to ‘learn more’, we can say that pretty much everything in a website that demands user interaction to be displayed is a call to action button.
If the user is interested, he will click to find out more. Using only colors or bold letters is not the main concern here, you need to think about user experience, positioning the button in the proper place and using the correct words… and most important, you need to know what you need to prioritize.
So, I gather some examples of buttons like these to show here. From clean and direct layouts to colorful and creative ones, these are good examples of call to action buttons.