Groups in ATLAS.ti are filter devices.
Document groups can be regarded as attributes or variables. It is possible to combine them using logical operators like AND and OR, for instance to retrieve and analyze data from female respondents living in NYC. This would an AND combination of two document groups.
Code groups serve as a filter for codes. They can for instance be used to filter all codes that need to be aggregated if you have been coding very detailled and in the process of generated too many codes.
Code groups are not a kind of higher order code. It is NOT recommmend to use them as categories. For further information see Building a Code System.
For more information on groups, see Working with Group.