Dedicated server is a service where the user leases the entire physical server and is not shared by anyone else. It is literally just a full physical server, dedicated to a single client. Unlike VPS the user has full control over the server and its hardware. The user can choose the hardware, software, OS, etc. Dedicated server is more expensive than VPS because of the benefits like high performance, security, email stability, uptime and full administrator control.
Having a Dedicated Server is like renting a house. You are entitled to all the resources available within the house; no one else will be competing for the resources that you own.
