A Howitzer is a powerful, automatic loading artillery cannon. The current artillery in the USA military is the M109 Howitzer. It is mobile and can fire many, high explosive rounds at far away targets. Howitzers are generally used for support or to soften up an area before said area is invaded.
Before the massive assault on Fallujah during Operation Iraqi Freedom, starting in November of 2004, Howitzers fired many rounds at strategic targets to soften up the area, then fired during the invasion when soldiers needed support.