Charleen, Basically they are just two closely related groups of plants. Lynn mentioned one clear feature that helps to distinguish the two.
Sempervivum have a broad range of how long the stolons are. They are also identified by characteristics of the flowers, but basically its a thing with the genetics. There are a number of species of Sempervivum. For example, Sempervivum graceum, Sempervivum octopodes, Sempervivum montanum, etc. There are a lot. The more colorful plants we have in our collections tend to be hybrids that have been developed over time and enhanced from selective breeding.
Jovibarba is another family of plants. They are closely related to Sempervivum and, in fact, have been placed in Sempervivum at some points in time. I think at this point Jovibarba and Sempervivum are separate families, but the scientists could change that at some point. Jovibarba also have different species. There is Jovibarba allonii, Jovibara hirta, Jovibarba heuffelii, and Jovibarba sobolifera. Not as many as Sempervivum. A lot of work has been done with heuffelii over time. Just like with Sempervivum, there are a lot of different varieties that have been developed with selective breeding by hobbyists.
Jovibarba have different genetics than Sempervivum and generally it is very hard to cross breed the two families (I do not know if this has been successfully done with stable results or even possible). As a rule of thumb, Jovi's have fewer petals then Semps and their flowers are yellow and white. Semps have petals in different colors, generally with a range of reddish hues, but there are some that are yellow.
