Horde Levels 12 to 20.

In a word, The Barrens.

This is the largest and the busiest experience zone for the Horde. You really can go all the way from level 12 to 20.

Start at the northern end, where the mobs are 12, and work your way down south where the mobs go up to level 24. Quests are available from The Crossroads, Rachet and Camp Taurajo.

I won't repeat the information available in many of the excellent guides available, this is a good Guide to The Barrens. Another here. One with a good map here.