Most sliding doors have a interlocking strip between them in the middle where they overlap when closed. This helps with the strength of the door and keeps the weather and prying tools out.
Most vinyl doors have a lock or latch that doesn't work well, thus putting a stick between the door and side jamb.
Sliding doors have rollers on the bottom of them and travel on a track. If the sliding door isn't adjusted to the right height then they can be lifted up and out. A simple adjustment can fix this, click 'How to Raise a Sliding Glass Door'.