Booster seats come in several varieties - high back, backless, and harnessed. There are also "all-in-one" seats that can be used from five to 100 lbs.
High-back and backless belt-positioning boosters: "boost" the child, helping to properly position the vehicle’s lap/shoulder belt.
Harnessed boosters: use an internal harness restraint system to secure the child. As the child grows, the harness system can be removed - with the seat converting then to the belt-positioning mode.
All-in-one car seats: literally grow along with the child. They can be used in both rear- and forward-facing positions and later as a belt-positioning booster.