Quick answer: All Star cheer is competitive cheerleading performed by teams based at private gyms rather than schools. Athletes train and compete for the gym's team, year-round, purely as a competitive sport rather than in support of another team.
All Star vs School Cheer
School cheer teams are usually attached to a specific school and often combine sideline support for other sports with some competitive events. All Star teams, by contrast, exist purely to train and compete - they aren't tied to any other sport, and athletes typically join a specific gym rather than a school.
How All Star Teams Are Organised
An All Star gym usually runs multiple teams at once, organised by level and age group, so that athletes of similar age and ability compete against comparable teams. A single gym might field a Level 1 youth team alongside several teams at higher levels.
Training
All Star teams typically train several times a week year-round, building a single competitive routine that's refined and re-performed at many events across a season, rather than changing routines week to week.
Where All Star Teams Compete
All Star teams compete at events run by competition providers, sometimes working toward qualification ("bids") for larger end-of-season championship events. See What Happens at a Cheer Competition? for what a typical event looks like.
If Your Child Is New to All Star
Start with Cheer Levels Explained to understand how teams are grouped, and the Parent's Guide for a broader introduction to the sport.