Difference between OU and group

Organization unit :

OU can be used to segregate/filter department bases on the region or type of users/groups/computers. OU - Structures are included to make Administrator job easy. For eg- You have 2 office 1. Main office 2. Branch office ,

You want to separte users and computer accounts for respective location so that you can go ahead and manage them easily , then you need to create OU structure in AD and move the user accounts or computer acccounts respectively according to your need.

Groups :

Groups can be used to group to be able to apply permission instead of doing it one by one. Its easy to manage group then individual. Consider, you need to add 1000 users in a folder but instead of adding one by one, you can add a security group & later on you can modify group to add or remove users instead of going to folder & adding or removing it manually.