What is VPS .
It means: Virtual Private Server
Virtual Private Server technology presents multiple isolated server environments on a single physical server which share same hardware, licenses and management effort with maximum efficiency.
Each VPS is completely independent for eachother. It uses its own RAM, CPU, IPs, disk space, etc. If one VPS on the server is being attacked (by a DoS for instance) all the other VPS' are completely unaffected since they have their guaranteed (dedicated) amount of resources. VPS' cannot affect eachother in any way. In nearly every perspective you can see a VPS as if it's a dedicated server.
What makes VPS technology so unique?
Independency - Having complete root/administrator access gives you ability to install and run your own applications, ability to manage your own configuration files and ability to fully control all files and processes on your VPS. Furthermore, no matter what happens you will remain your guaranteed amount of resources.
Scalability â€“ You can easily upgrade your VPS according to your needs with no downtime at all. Even if you have reached our largest plan we can still upgrade you to a larger plan.
Flexibility and efficiency â€“ a VPS can perform the same as a dedicated server, yet it's a much smaller investment.
The positives were mentioned by Blue, it is cheaper than a dedicated server, and it is easier to learn on that a dedicated server. You can also have root access to the server, so it is better than a shared server, and there probably aren't as many accounts on a VPS machine as there are on a shared machine.
The negatives are, you share your resources, suach as CPU, RAM, B/W. This can cause bottlenecking. If you are planning on hosting something which requires high bandwidth usage, like video streaming, then VPS might not be the route you want to take. Also, with all of the users rooting in, and sharing your resources, your sites may be sluggish at times, and it may also go down because of someones else mistakes. Another thing is, because it is manage, you are not as free to do what you want as far as updates on software and other things, as you would be with an unmanaged dedicated server.