What is the difference between a Server OS and Desktop OS ?

Server platform can provide features like:

1) unlimited user connections
2) use of large amounts of memory
3) can act as web server, database server, email server and other server-like roles
4) optimized for network, instead of local application execution
5) extended management
6) extended fault tolerance to avoid downtimes
7) can hold a domain
8) expensive

Client platform can provide features like:

1) running client applications faster, like Office, Photoshop, Games
2) easy access to web services, like email, browsing, searching
3) rich media services
4) easy to use for non-expert users
5) can work on a domain as a member (professional edition only - home edition cannot join a domain)
6) rich connectivity support (lan, wireless, bluetooth, etc)
7) cheaper than the server version