When using Advanced Search, if you select several groups in the Group list near the top, it will treat the search as an OR search, and return results for contacts who are in any of the groups you select. If you want to find contacts who belong to all of the selected groups, you will need to use Search Builder.

There is also a very useful built-in custom search, "Include/Exclude Contacts in a Group/Tag", that enables you to find contacts who are in one group but not in another, which you can find by going to Search > Custom Searches in the navigation menu.