When to ask for login?

1. What is the purpose of login?
2. Is login and Registration same?

What is the purpose of login?

In general computer usage, logon is the procedure used to get access to an operating system or application, usually in a remote computer. Almost always a logon requires that the user have (1) a user ID and (2) a password.

Is login and Registration same?

Registering for something sometimes involves paying money, while signing in or logging in to something usually does not require a payment.