There are three main types of cat flap, magnet flaps, manual flaps and microchip cat flaps. Magnet cat flaps have a simple magnet to help keep it closed and the weather out.
Manual flaps swing open and closed as the cat passes through but has no magnet to hold it closed. A microchip cat flap can be programmed to only allow your cat to pass and lock for all others.
You can also get variants of those three cat flaps like patio door cat flaps or glass door cat flaps. These are designed specifically to sit flat against the glass and to allow sliding doors to open normally.