a dedicated server means you have use of the entire server and all of its resources. A VPS server means that the 1 whole server is seperated into maybe 8 different Virtual Private Servers and those 8 share the whole servers resources. So if someone on abcwebhost.com's VPS runs a ton of processes and bogs the server, it still effects your clients. A dedi server wouldnt do that because only you and your clients use it.