Think of a shared server as a large apartment complex, and all of the individual apartments are rented by other website owners. All of you need a place to live — just like your website’s files — but going out to buy a huge family home would be too expensive for your needs. Sharing common areas and utilities in an apartment block helps keep costs down. And the same is true for shared hosting.
Similarly, in the case of VPS, there are several users that are using the same server but they are isolated from each other. It means that no one will be affected by how much resources another is using. You will get the speed and security that you need without compromise. It is almost a perfect scenario because you will get the benefits of a private server with shared cost of services.
Essentially, here’s the criteria I would use to judge things – if your site is made up of primarily static, HTML-based content, then you probably don’t need a VPS package. However, if you have a large amount of files stored, multiple sites, dynamic content, and the possibility of major traffic from time to time, then you might consider upgrading to a VPS. It’s a powerful package that allows you to do more than you could with a shared hosting plan, but requires less investment than a dedicated server.
You’re paying more, so there SHOULD be a minimum uptime guarantee and better server speed. Look for a host that offers 99.5% at a very minimum, although ideally, I’d rather go with someone who offers 99.9%. Search through some reviews as there are many who have put this to the test. For example, any of WHSR’s many web host reviews include an uptime record as one of our key tests.
In answering this question, maybe it’s better to examine how VPS hosting fits in to the overall offerings of most hosting companies. Shared hosting is just that – your site is hosted on a machine with a bunch of other sites, and each of you share the same resources, including RAM, disk space, and CPU. Your site uses what it needs if it’s available, and if it’s not – well, that’s the limitation of shared hosting. Likewise, a dedicated server is also self-explanatory – your site is the only one hosted on server, and you have all the aforementioned resources available at your beck and call. Dedicated hosting is therefore for those large sites with big databases and lots of traffic, whereas the limitations of shared hosting’s usually prevent it from housing that kind of site. Dedicated servers are also relatively expensive, while one can get a shared hosting plan for under $10 per month.