Consider the illustration shown. A generic amplifier is often described as a "Black Box". It has an input (port) and an output (port) with two terminals each.
As you have seen, a transistor is an amplifier, but has only three terminals. How does this go together?
A transistor circuit therefore has one terminal which is common to the input and output. Thus, you automatically obtain three possible transistor circuits...
But are these really amplifier circuits? If so, how do they differ, and how are they used...?
In the further course you build these circuits and investigate their characteristics...