VPS :-Virtual Private Server (VPS) technology presents multiple isolated server environments on a single physical server. VPS allows multiple customers to share the expense of hardware and network connections without sacrificing privacy, performance or preference. Each VPS is completely independent from each other and it can be rebooted independently and has its own root/administrator access. VPS provides the flexibility to add and change modules and install your own software. Semi-Dedicated server :-Semi-dedicated server acts exactly like a VPS (virtual private server). It will also provides you root and administrator access. As compared to a VPS Hosting this Semi-Dedicated server is shared between only 6 users which will allow you to use more CPU resources and large amount of Bandwidth. Semi-Dedicated server is very ideal hosting solution for those who need more control and resources. Source - Prilep
^ Semi dedicated does not necessarily mean sharing between 6 users but is usually much lesser than that of the vps systems.
Typically semi-dedicated/hybrid servers give you a full core, you will want to ask the hosting company what they exactly mean by semi-dedicated.
I think semi-dedicated will not give you root access unlike with VPS you have root access and can do things on your own