Small group training usually occurs when you have between 2 and 10 participants focused on the same goal.
In most group settings everyone does the same exercises together. In other settings participants move from station to station for set periods of time. Most groups will be made up of individuals who are on the same fitness levels, this allows for more camaraderie and support from other members of the group.
Group training provides a less expensive alternative to personal training with many of the same benefits.
Because the trainer is working with more people at one time, he or she can afford to charge each individual less for the same hour of raining.