What is the basic difference between local, global and universal groups in Active Directory? What is the best practice when assigning users and groups to these groups?
The difference is which groups/users you can include in the different groups, which permissions can be assigned to that group, and if the group can be converted.
If you only have one domain and one tree, and you know it will stay that way forever, you really don't need to know a lot about this. But, i strongly encourage you to read this: http://technet.microsoft.com/en-us/library/cc755692%28WS.10%29.aspx
You will hate the person who set up groups in the first place if you ever have to fix their mistake. I've had to, and it is no fun.