The Browns are replacing Horton with Rams' defensive coordinator Gregg Williams. Williams has been a defensive coordinator for most of his career (1997-2000 Titans, 2004-2007 Redskins, 2008 Jacksonville, 2009-2011 Saints (where he won a Super Bowl in 2009), 2012, 2014-2016 Rams. And was a head coach once from 2001-2003 with the Bills. This past season the Rams' had the ninth best defense only giving up 337 yards per game.
3-4 defense (three lineman four linebackers) but Williams runs a 4-3 (four linemen three linebackers). The change to the 4-3 should help the Browns going forward especially since the Browns have the number one overall pick in the draft and could potentially take Myles Garrett who is a 4-3 defensive pass rusher.
Per Mike Silver of NFL network: Hue Jackson wanted Williams because "he can adapt scheme to fit talents of his players."