The standard grips this chord is usually shown with.
Em is E, G and B, a minor triad. The shape this page opens on is written 022000, lowest string first.

Also written Emin, E- or E minor. The same chord either way; which spelling you meet depends on who wrote the chart.
There are 10 common shapes for Em on the guitar, and 54 derived voicings in 2 families.
In which keys. Every note of Em 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: 022000, 022453, xx2453, xx5453, x7545x, x79987, x-10-9-9-8-x, 12-10-9-9-x-12, 12-14-14-12-12-12, x-x-14-12-12-12.
Triad. Three notes across one string set. Em has xx200x, 322xxx, x220xx, xx2000, xx2003, 0220xx, 3220xx, 3224xx.
Open string. An open string left ringing against a fretted grip. Because it costs no finger, it sounds a note the closed shape has none left for -- a tension on top, or a pedal root underneath. Either a familiar grip with open strings added, or a shape so sparse the open strings carry the chord. These belong to their key rather than to a shape, so D, A, G and E have many and Eb a handful. Em has xx2000, 0220xx, x7500x, 0x200x, 0x5000, 02500x.
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.