1.) Base Transceiver Station (BTS)
2.) Base Station Controller (BSC)
3.) Mobile Switching Center (MSC)
Picture shows how are they interconnected with each other and this is how a basic Wireless architecture looks like. Each entity has a specific role to play when you place a call or when you receive a call on your MS i.e Mobile station which is nothing but your Cell phone.
The complete architecture consists of few other network entities which too have some specific role to play during your call setup procedure. The pictures below shows complete architecture of a GSM network.
4.) HLR : Home Location register
5.) VLR: Visitor Location register
6.) EIR: Equipment Identity register
7.) Auc: Authentication center
To understand their inter working and role each entity plays during a call setup you need to study Basic Call Flow Process. " What happens when I hit the calling key on my cell phone? "
Have a look at the presentation for an Overview of a Cellular System: