Most newer car and car-seat models use the LATCH (or Lower Anchors and Tethers For Children) system for car-seat installation. The LATCH system attaches the car seat to the vehicle through anchor points installed in the car and connectors on the car seat, which makes for easy installation and can eliminate potential errors resulting from installing a car seat with a seat belt. It's important to note, however, that the 65-pound weight limit for using the LATCH system includes both the weight of your child and the weight of the car seat itself, which usually average between 20 to 25 pounds.
If the combined weight of your child and their car seat exceeds 65 pounds, parents should use the seat belt system and the car-seat tether to secure the seat rather than the lower anchor connectors. Seat belts are just as safe (and, at higher weights, safer) as LATCH, as LATCH was created to make for an easier install, not a better one.