Preorder relation is reflexive and transitive only, whereas partial order is anti-symmetric as well.
In case of preorder if you follow arrows you may come back to your starting point, but in case of partial order you can only go in one direction (apart from following identity).
It seems that if for each cycle of preorder you _collapse_ it to a single object you will end up with a partial order. From that perspective partial order is isomorphic to a tree, and pre-order is a _kind of tree structure with some local eddies allowed_.