ubuntu - Difference between sudo group and sudoers file?

I am creating a user in Ubuntu 14.04 using command adduser.

I want to give it sudo access. For doing this, I am consfused in between two ways.

First way is : If I add that user to sudo group using command usermod -aG sudo.

Second way is : If I edit the sudoers file using command visudo and add a line ALL=(ALL:ALL) ALL.

Please clear the confusion between these two.