Generally speaking, a semi-dedicated hosting gives you most resources (as RAM or CPU) containing the costs: this is made available exploiting some virtual server features and configuring hardware and software as well.
Using Supporthost this means that: you will have a cheap shared hosting with a limit (until 70 users) on the maximum account number on a single server. So you can say i.e semi dedicated hosting is like saying shared hosting with limited websites for giving better performance. In other ways, you could reach same results limiting directly CPU or RAM on the host you are exploiting for websites hosting, but this appears slightly more complicated (and less cheap).
If you need to host more than a website, semi-dedicated hosts could be the best for you.
Semi dedicated hosting is virtually between shared hostings and dedicated ones: with reasonable costs you can have a fully-functional shared hosting more efficent, affordable and good for your webservices and websites (blogs, forums, community and so on). Supporthost gives you one of the best services for all you web needs.