The first remote controls were created in World War I and used to trick German navy ships into crashing by confusing their radio signals. Then in WWII they were used to detonate bombs. When they began to be used to operate TVs, the very earliest versions had to be attached to the set, so they weren't really remote.
When you press a key on your TV remote, the pressure causes the remote to create an infra-red signal that is then sent to the TV. This is done through a chip, or integrated circuit, on the remote's circuit board. This chip detects when a key is depressed and translates it into a type of code, with different keys creating different code signals. There is also a transistor in the circuit board; the signals are sent to the transistor, which amplifies them and sends them to the infrared LED at the end of the circuit board. The infrared light created by the LED is then detected by the sensors in the TV, which then send a signal to change channels etc.