Synchrous allows direct editing of the model body with no history between operations. It has Live Rules which will dynamically apply relationships to geometry when editing for those typical logical relationships one would assume.
Ordered is your typical history based environment where a "recipe" is built and dependencies are rquired (Plane -> Sketch -> geometry... repeat).
Ordered is like a history-based modeler, where each sketch and feature creates an entry in a list of order-dependent, parent-child type relationships. To edit a feature, you have to go back and change options, settings, dimensions, or sketches within the overall definition of a feature. With Ordered you use sketches and features to build "design intent" - how you want the model to react to change. Synchronous enables you to control the model geometry directly with dimensions and relationships, and other tools. With Synch, you determine design intent at the time of the change.
Solid Edge uses both because each method on its own has limitations. There are things you can do with Synch that can't be done easily with ordered, but synch also has some limitations which are handled nicely in ordered, such as certain topography changes.