A 5K is a 5,000 meter race, which is 3.107 miles.
When it is a cross country or road race it is usually referred to as a 5K. When it is a track event it usually is referred to as a 5,000 meter.
It is very popular because it offers a challenge to new and experienced runners alike. It requires endurance to run the distance but is not so long that it becomes an endurance race. The distance results in an emphasis on strategy and pacing.