Register and enroll describe the act of gaining access to a service, or using a service for the first time. Usually, you are expected to give something in exchange for access to the service, such as your name, email address, or money. Login and sign in describe the act of accessing a service that you have previously enrolled in. As a verb, "login" can also be "log in," with a space between the words. "Login" is also a noun. It is usually the name or email address you use to access a service. To log in to Italki, you must input your login and password.