For a high level overview of LoRa this introduction might help.
Far more detailed information is available in the Semtech application note LoRa™ Modulation Basics.
LoRa broadcasts the transmissions. Literally anyone with the right hardware can receive the signal. So if you are in range of a gateway it will receive the transmission. TTN connected gateways forward all data to the back-end so if you transmit while in range of two gateways you will get a packet from both gateways in the 'old' API (duplicate data with different gateways). The new API will show the data once with a list of gateways that received the data.