With shared hosting, you can host multiple websites without the need of seperate hosting packages
But it all depends on the amount of space and bandwidth you require. If you think your shared hosting package has enough bandwidth and space for each website you're hosting - then you'll be okay
Semi-Dedicated hosting is almost like having your own dedicated server. You have your own OS installation with your own root access to your semi-dedicated package, and will be able to change PHP settings, Apache settings and software versions as per your request.
Semi-Dedicated is exactly like a VPS package, except there will only be 4 semi-dedicated setups on one server, whereas with VPS hosting, there can be up to 20 setups on one server.
Semi-Dedicated/VPS are usually for those who are more technically mature and know how to manage a server effectively. However, eUKhost VPS/Semi-Dedicated & Dedicated Server packages are all fully managed.