Difference Between Sign In and Login (Users and Groups)
I think arguing about login vs sign-in is sematics.

Sql server allows you to log in via 2 modes.


SQL authentication: means using a user id and password which is not the same  as your domain credentials. 


Or.


Windows authentication: This can be granted to an individual domain user or a domain group. 


Domain groups are a nice way of granting access without having to grant access to individual users each time. For example if a department or team should all have access to a system (e.g HR or Payroll) you can grant access to the team as a whole. As users are added to the team in AD they automatically inherit access to the db via their membership of the group.

This can be overridden by denying individuals access if required. Eg all of HR have access except user xyz.

If SQL is configured to use both modes this referred to as "mixed mode" authentication. 

A login gives you access to the SQL  instance. You also need to be granted access to individual dbs. Once you have access to sql the method becomes irrelevant. Once you're in, you're in. 

Technically there is third method related to contained databases. But in my experience this is rare.