PayPal can be added to WordPress in a variety of ways. The simplest being just adding a PayPal button on your website using their interactive button generator where you can copy and paste the code they give you onto a web page. Another method is to use PayPal plugins, these come in many forms and always come with additional functionality. For example, if you want to accept PayPal payments but also want to book appointments, you could use Simply Schedule Appointments as your PayPal plugin.