I mostly work with EPiServer and Sitecore, and I can tell you the difference in short:
Sitecore has broader architecture and more powerfull UI. CMS is deeply configurable and highly extensible, it has clever publishing and caching system, powerful search and page editor. But it doesn't provide much out of box and UI is pretty old, slow and hard to learn. So this will be a long journey until you understand it good and make a good support of all its features for editors.
EPiServer is easy, friendly to users and developers. It provides an essential bunch of features out of box, has easy UI and page editor, good drag-and-drop experience, easy personalization. It is code-first, distributed with NuGet, provides dependency injection for its services, out of box MVC support. But it's not so extensible and configurable, has pure search (without expensive EPiFind module) and generally lower-featured comparing to Sitecore. So it's good for small/middle websites, but can be an obstacle in complex solutions.
Both have similar tree-item concept, rich documentation, pure public module system and hard UI customization. Both expensive and not open source.
As I know, Umbraco is pretty similar to EPiServer and Sitecore, but free and open source. Of course you get less features, more bugs, not much docs and no free support.
Orchard is really different comparing to other three CMS. It is module-based like Wordpress: you use standard or public modules and themes, instead of writing the whole website from scratch. You create your own themes and modules to customize the website and CMS. So entire CMS is highly extensible and provides a lot of free community modules. But in the same time you lose control and learning curve is much longer. Orchard is free and open-source, entirely MVC-based, UI and API are well done, but it can be hard for both developers and editors to understand it.