Noob question about type of rifle

Howdy-

Complete newbie with a newbie question. I have been given a "Mendoza 900". It is 4.5mm and uses two parts to fire; a 4.5 pellet, and a small "rimfire" looking cartridge behind it. Hammer drop, single shot. 

Can someone please tell me what the name of this type of firing system is? Is it a true "airgun" or what class does it fall into? 

I can find nothing on the Google.

Thank you- David