To "log in" to a computer means to enter your user name, email address, password, and other information to get access:

A: How can I help you?

B: I need to pay my bill but I wasn't able to do it online.

A: OK. Were you able to log in?

We combine the words "log" and "in" to form "login". Your "login" is the information that you use to log in to a computer. Usually it means your user name:

I forgot my login and password.

Other times, it might mean both pieces of information:

Here's your login. Keep it safe!

We can also use "login" like this:

Do you remember your login information?

The word "login" can also mean the action of someone logging in to a computer:

After 3 login attempts, they lock your account and you have to call them to get it reset.