What you meant by Solver in Houdini ?
Can anyone help me with details .

In DOPs using dop objects like flip object, rbd object, smoke object etc. we create necessary data like physical properties as mass, friction, density etc.
These are just objects with holding data. For an example, we have any one of dop object and dop force like gravity. Now we need to affect dop object by gravity. But these are just data which don't know to interact with each other. Now we need SOMETHING to talk with that data. That something is called Solver in Houdini. It just like brain in human body which give command to other body parts to do required action on certain time.
In short solvers are OPs (They exist in DOPs) which solve physical property (data) of objects.

I suggest you to go through this:

http://en.wikipedia.org/wiki/Solver [en.wikipedia.org]

thx…