A single-cylinder has an inside knob and an outside keyhole. This means that no key is required to unlock from inside. Nevertheless, an intruder breaking in through other means will be able to walk out through the door.
Using single-cylinder type on storm doors with glass is not advisable. This leaves an easy way for burglars as they can break the glass and turn the knob inside.
In a double cylinder type, a key is required to turn the bolt from either side. This defeats the security concerns of the former. Self-locking double-cylinders are a double-edged sword. In case the key is missing during an emergency, the door has to be destroyed for gaining access