The brad nailer is attached to an air compressor and can shoot brads of any size from 3/8" to 1-1/4". This sort of air nailer is good for smaller projects such as attaching lightweight molding or cabinet backing.
The finish/trim nailer is used for larger projects and for attaching molding. It's driven by an air compressor and shoots 1-1/4" to 2-1/2" nails.
The portable finish nailer has an internal combustion engine, a gas cartridge and a battery, and thus needs no air compressor.
The portable framing nailer is also driven by an internal combustion engine, a gas cartridge and a battery. It can handle bigger nails — up to sixteenpenny — and makes jobs such as framing go a lot faster.