Common Source is an FET configuration in which the source is grounded and common to the input and output signals.
The common source circuit provides a medium input and output impedance levels. Both current gain and voltage gain can be described as medium, but the output is the inverse of the input, i.e. 180° phase change. This provides a good overall performance and as such it is often thought of as the most widely used configuration. This FET configuration is probably the most widely used.