Both the B1 and the B2 visas are temporary, nonimmigrant visas. But, like some of the other answers point out, the B1 visa is for business while the B2 visa is for pleasure. A B1 visa will allow the holder to visit the US to work on some business-related project. A B2 visa is basically a tourist visa and allows the holder to travel to and within the US for a pretty broad range of tourist activities.
Sometimes, a visa will be issued as a B1/B2. This just means that it is both for business and for pleasure. A lot of times when a foreign national enters the US for a work-related project, they will also want to do some tourism, and this combination visa will allow that flexibility. So, to answer your specific question, how is a B1/B2 different from B1 and B2 visas? It isn’t. It’s merely a combination of the two.