By default items are grouped in hierarchic layout to show the project structure. This is a multi-level grouping option where items are grouped under their parent object (ie. project, sequence, shot). Shots are grouped by sequence, and so on.

Other grouping options are Shots/Asset builds and Tasks layout where the project structure is flattened. (Top left in the screenshot)

It is also possible to group tasks by their attributes, eg. assignee, status, type, a custom attribute.

Some of the attributes are summed at the group level.