These linear actuators are self contained units that include an electric motor and quadrature encoder.
They will push up to 25N and move at 12mm/s unloaded.
For use with your NXT® Brick you will need a standard NXT® connection cable to connect it in place of one of the Mindstorm® Servos, and you will need to download the Firgelli L12-NXT Block (see Downloads).