You can choose one column to sort by on your report. If you are viewing the report in an item list on the fly, you can always change and/or add additional sorts. Not all columns are sort-able, however, the ones that are, can be set as a default for your report. Check the 'sort by' box next to the column you want to sort by and choose the direction, ascending (A-Z, 1-9) or descending (Z-A, 9-1).
When you choose to group by a column, we will group all data with the same value for that column into one row of data. If that data for the row is able to be summed or counted or averaged based on the defined column, then we will do that. If it's text data, we will only show the first value we find in the group.
Example: You would not want to group by customer type and have organization as a column. You would instead have something like Lead $ Total that we could sum. This would give you the Lead $ Total for each customer type.