What is the purpose of Windows Server?

Microsoft Windows Server OS (operating system) is a series of enterprise-class server operating systems designed to share services with multiple users and provide extensive administrative control of data storage, applications and corporate networks.

Can I use a regular computer as a server?

Pretty much any computer can be used as a web server, provided it can connect to a network and run web server software. Since a web server can be quite simple and there are free and open source web servers available, in practice, any device can act as a web server.