We should be aware that there is a difference between what is ment by a "buffer" in the contexts of digital and analogue electronics.
In the analogue domain a buffer is an amplifier having a unity voltage gain but, potentially, a current gain over unity.
In the digital domain, a buffer is a logic element with one output whose locic state is identical to that of its single input (although devices such as "tri-state buffers" could be regarded as exceptions). In practice they are generally expected to drive lines or large fan-outs, provide level translation, etc, and so may exhibit both voltage and/or current gain.
Conceptually, both analogue and digital (logic) buffers fulfill similar roles, but electrically they may be very different circuits.
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Regarding the circle, what dougy says is exactly right...
Bare in mind that you might see the circle on inputs as well as outputs, indicating that the input is inverted.