Video by Google for Education via YouTube
Source

To use Classroom, you sign in on your computer or mobile device and join classes. After you join a class, you can get work from your teacher and communicate with your classmates.

You can join a class with:

A class link—Your teacher sends you the link.
A class code—Your teacher sends or tells you the class code.
An email invite—Your teacher sends you the invite.
After you join a class on one device, you’re enrolled in that class for all devices.

To join a class, be sure to sign in to Classroom with the correct account.

Go to Source