User groups enable you to group tests in a logical
order.
The following schedule contains one user group.

If
you run this schedule with 10 users, they are assigned to the only user group—Browsers
and Buyers. When the schedule run starts, the 10 virtual users start executing
the first test in parallel. As soon as one is finished, he moves to the second
test. Thus you have ten virtual users, all starting at the same time and executing
each test sequentially. This does not give you much control over the run.
The following schedule contains the same tests in the same order,
but they are divided between two user groups. Conceptually, this schedule
is easier to understand, because the user's tasks are grouped logically—the
Browsers browse and the Buyers browse and then bid on a product. But, even
more important, this schedule is a much more accurate representation of the
types of users on your system, because each user group contains tests that
represent the actions that they do, and the proportions of the user groups
(70% and 30%) represent the proportions of the users on your system.

If you run this schedule with 10 users, seven are assigned to the Browsers
group, and three are assigned to the Buyers group. When the run starts, the
seven Browsers and the three Buyers start in parallel. Thus you have seven
Browsers, each executing two tests sequentially, and three Buyers, each executing
three tests sequentially.