There are two main kinds of buffer circuits, Voltage buffers and Current buffers. The purposes of each is to isolate the mentioned characteristic to avoid loading the input circuit or source from the output stage.
Another name by which buffer amplifiers are known as is a voltage follower. The name is given because of the characteristic of the amplifier to output a signal of the same amplitude as the input (given the unity gain [gain of 1 or 0dB] ).
Examples of Buffer amplifiers:
The examples are too many to mention in this page, the most common being the transistor voltage follower and op amp version of it. The exact characteristics, formulas and construction instructions can be found on the specific component's page.
