Windows 10 Pro is designed as a desktop operating system while Windows Server is designed as a server (as the name implies) however can also be used as a desktop option.
Windows Server include server specific software such as Server Manager which allow you to use server features such as IIS web server, active directory, dns server, etc. Windows server also has less installed features and programs by default. For example you won't find Cortana, Microsoft Edge browser, Microsoft store, games, etc resulting in more free disk space available. A basic install of Windows 10 Pro will use about 20 GB of disk space while Windows Server 2016 will use approximately 12 GB of disk space.