Selective racking gives you a reach into every pallet position but spends most of its footprint on aisles. Drive-in and push-back racking flip that trade: pallets store several deep in a single lane, with one aisle serving the whole lane instead of one aisle per pallet. The result is a lot more storage density in the same building footprint, the cost is that you can't reach every pallet directly, only the ones at the front or back of a lane.
The two systems store deep differently. Drive-in racking lets a forklift drive directly into the lane on rails to place or pull a pallet, first-in-last-out. Push-back racking stays out of the lane entirely, pallets ride nested carts on an incline, and loading one pushes the row behind it back, unloading pulls the next one forward. Push-back gives you 2 to 5 pallets deep without the forklift ever entering the rack; drive-in typically runs deeper but needs the truck inside the structure.