Blinded
When blinded, a creature cannot see. Many checks may be impossible for the blinded creature (like using a perception check to spot someone in a crowd).
A blinded creature has disadvantage on attack rolls and dexterity saving throws.
Attacks made against a blinded creature have advantage.
A blinded creature's speed is halved.
A blinded creature cannot make attacks of opportunity because it cannot see the creature moving away.
A blinded creature does not have line of sight for spellcasting.
Notes
The blinded condition in 5e is not always intuitive. An archer can attack any creature they want while blinded, only suffering disadvantage.
A blinded creature knows where other creatures are for the sake of moving closer to them. The GM may rule that a blinded character is unaware of things in their environment beyond what the blinded condition prescribes.
A GM may wish to rule that a blinded spellcaster can occassionally cast spells that normally require line of sight. We recommend allowing blinded casters to cast such spells when in physical contact with the target, such as when the caster is grappling an enemy, or when the spellcaster is swallowed by a large creature.
An example of the blinded condition interacting with blindsight:
An Adult Red Dragon has a blindsight of 60 ft. This means if the dragon is blinded, a fighter attacking the dragon with a melee attack does not get advantage on the attack roll, because the dragon can see the fighter. However, an archer 100 ft. away does have advantage on attack rolls against the dragon because the dragon cannot see the archer.