See the General Rules (wef. 26 Sept 2013) and FINA Swimming Rules (wef. 25 Sept 2013).
You may also read the comments on the changes from swimswam.com, where it's worth noting the excerpt below :
"In breaststroke and butterfly, two-hand touches can no longer be overlapping. The hands must be separated when the wall is touched.
In backstroke, FINA took out the wording that a swimmer is allowed to be completely submerged at the finish of a race (though they can still do so at the turn). This means that the big lunges we see, most often in younger age groups, that put the swimmer entirely underwater into the wall are now eliminated. This is important for all coaches to keep an eye on."