Em/G is G, E and B. The shape this page opens on is written 322000, lowest string first.
Note by note. G is the root, E is the sixth and B is the major third.

Also written Em over G or Em with G in the bass. The same chord either way; which spelling you meet depends on who wrote the chart.
There are 5 common shapes for Em/G on the guitar.
One note away. Change a single note of Em/G and you have E (with one note moved), Edim (with one note moved), Esus2 (with one note moved), Esus4 (with one note moved), Em6 (with one note added) or Em7 (with one note added).
In which keys. Every note of Em/G belongs to C major (the iii), D major (the ii), E minor (the i), G major (the vi), A minor (the v) and B minor (the iv).
The common shapes, written lowest string first: 322000, 322003, 322450, 10-10-9-12-12-12, x-x-12-12-12-12.
Under the hand. 3 of the 5 shapes here sit in open position, and the first uses 3 open strings. 2 of the 5 need a barre. Together they cover the neck from the 2nd fret to the 12th.
What kind of chord this is. Em/G is a slash chord — a triad over a named bass note. slash chords on every root, and what they are for.
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.