Base Transceiver Station (BTS) is the equipment in the GSM network which facilitates the wireless communication between mobile equipment (ME) such as cellphones and the network.
It contains one or more TRX entities.
Please note that a BTS is NOT an autonomous base station. It is only one element in a GSM network. You still need a BSC, MSC and HLR/VLR to run a minimal GSM network. OpenBSC is a Free software program attempting to do that.