From the BSA Cub Scouts overview:
Cub Scouting is a year-round, family-oriented part of the Boy Scouts of America program designed for girls and boys who are in first through fifth grades (or are 7, 8, 9, and 10 years of age). Parents, leaders, and organizations work together to achieve the 10 purposes of Cub Scouting:
Sportsmanship and Fitness
Fun and Adventure
Preparation for Boy Scouts
You can read more about Scouting in on the BSA website's Cub Scouts page.
Our uniforms page has some information on Cub Scout uniforms.