Let's depart again from the narrative to see how a transistor works. Transistor stands for transit resistor, the temporary name, now permanent, that the inventors gave it. These semidconductors, like the triode, control the electrical current flowing between two terminals by applying voltage to a third terminal. You now have a minature switch, presenting either a freeway to electrons or a brick wall to them, depending on whether a signal voltage exists. Bulky mechanical relays that used to switch calls, like the crossbar shown above, could now be replaced with transistors.
Transistors also amplify.
Like the triode described before, a weak signal can be boosted tremendously. Let's say you have ten watts flowing into one side of the transistor. Your current stops because silicon normally isn't a good conducter. You now introduce a signal into the middle of the transistor, say, at one watt. That changes the transistor's internal crystalline structure, causing the silicon to go from an insulator to a conductor.