These actuators are usually used to actuate central locking mechanisms in cars.
They use a motor with a rack and pinion to get the linear motion. Powering with one polarity extends the rack and reversing it retracts it.
Current draw is in the vicinity of 5Amps @ 12V DC. They are only meant to be powered for very short periods, otherwise they will overheat and burn out.
They are available as master (5 wire) or slave (2 wire) versions.
They are available from about $3 on eBay.
It was $7.50 on eBay.