Simply, a call to action is what you want someone to do when they get to your site. How you want them to interact with you or your business.
Having a website should mean that you are trying to grow your business and keep you current customers happy. If you are trying gain customers/users, you need to have them interact with your website. This is more then just browsing your homepage and leaving. This means getting them to engage, and that is where the CTA, call to action comes in.
The simplest CTA would be clicking on your email link or phone number on a mobile device. If this is all you want from you site, direct contact, then make these the first things you see on the site, normally at the top. Then, you need to make sure they are easy to click and do what they are suppose to. Always insure you are using HTML5 for these to call to actions, there is something so annoying about being on a tablet/phone and having to search out the ‘@’ key because the form box wasn’t built correctly.
Buy buttons are pretty self explanatory. This is a process that needs a full discussion in another post.
Feedback buttons or forms are a great way to keep current customers engaged. Giving value to their opinion keeps them interacting with your website and gives them an opportunity to spread the word.
Social media likes, reposts, pins, tags, etc are all simple and low commitment ways to get your visitors to engage. Insuring it is a simply on-click action to like or share your blog post, pin a lovely picture you’ve taken, or email to a friend, insures your message spreads.
Simply put the purpose is to gain a new customer. How you measure this gain is completely up to you and your type of business. You may just want to gather data about your visitors, you may want to increase your mailing list, or you may want to sell something. Each type of CTA above will give you good shot at doing this.