A VPS is a slice of a dedicated server.  However, just because it offers more power than a shared account does not mean it is the best solution for all cases.

WHEN SHOULD YOU USE A VPS? VPS's are great for medium projects which require a lot of processing power but do not really require a full dedicated server.  They are excellent for situations where you do not want to disturb other users on your server by spiking demands.  The amount of power and ram you can use is set on a VPS so you are able to use your share and not intrude on others.  They are also great for when you need absolutely ability to alter the shell or root.  Something you can't do on a shared server.

WHEN SHOULD YOU NOT USE A VPS? Because a VPS is essentially a virtual dedicated machine, you can mess it up by altering the root too much.  You are responsible for fine tuning your machine. If you are not an experienced webmaster, you should stick with shared (normal) hosting which is fine tuned by our staff..  A VPS also may need to be reset from time to time if your processor or ram use gets out of control.  You should be knowledgeable on how to restart the server (usually easy for the experienced webmaster).

A VPS is not ideally suited for use as a reseller account, i.e. selling hosting to others.  This is because it is not specifically load balanced for many different sites like a standard reseller site is.  It is made specifically for one webmaster to control and use.