C7/G is G, C, E and Bb, a dominant seventh over its fifth. The shape this page opens on is written 312010, lowest string first.

Also written C7 over G or C7 with G in the bass. The same chord either way; which spelling you meet depends on who wrote the chart.
There are 3 common shapes for C7/G on the guitar.
In which keys. Every note of C7/G belongs to D minor (the ♭VII) and F major (the V).
The common shapes, written lowest string first: 312010, 3323xx, 335353.
The common shapes and the derived voicings come from different places, and both are explained in where the chords and the theory come from.
Shows each chord the way this style would voice it, and marks which of its alternatives belong to the idiom.