Glyph of Warding is a level 3 abjuration spell. This spell allows spellcasters to inscribe a ward that can trigger various magical effects when stepped on by an enemy.
Description
Inscribe a circle of arcane glyphs on the ground. When stepped on by an enemy the selected magical effect will trigger.
The glyph will last until triggered, or until a Long Rest.
When the glyph is triggered, those affected by it each rolls a DexteritySaving throw. If a creature successfully saves, they only take half the damage for the damaging variants of this spell, and suffers no ill effect for the non-damaging variants.
Only those considered an enemy can trigger the glyph, for the player's party this is indicated via a red circle beneath the creature, red outlines when highlighted, and denoted by a red dot in the minimap. Neutral, friendly, and allied creatures (highlighted yellow/green/blue respectively) can safely go through the glyph.
This makes using the glyph as a trap difficult for many encounters. Specifically, those that normally only consider creatures hostile after dialogue.
If the glyph is cast in an area where hostile creatures are already present, it triggers immediately.
When the glyph is triggered everyone within the area is affected, regardless of allegiance.
Only one glyph can be active at a time.
If another glyph is placed while one is already active, the previous one will fade and vanish with no effect.
The limit is one per character; multiple characters can each have a glyph active at a time.
When the spellcaster is surrounded by enemies, sometimes it is worth throwing the glyph down right at their feet. This is true for most AOE spells but it is especially so for the sleep variant of Glyph of Warding:
The shared Initiative system means an ally can immediately shove/help the spellcaster awake should they fall asleep due to their own glyph.
Only one glyph can be active at a time.
Level 3 Spell Slot
Range: 9 m / 30 ft
AoE: 4 m / 13 ft (Radius)
Dexterity 
Saving throw. If a creature successfully saves, they only take half the damage for the damaging variants of this spell, and suffers no ill effect for the non-damaging variants.