Yeast is a leavening agent for baking. It can also be used for fermentation to make alcoholic drinks. Yeast is a single celled micro organism in the domain Eukaryota and Fungi kingdom. Saccharomyces cerevisiae is the species that is most frequently used for backing and brewing. Because it's the species used for these purposes, it's also known as baker's yeast or brewer's yeast. There are other species in the Saccharomyces genus that are used for brewing.
Yeasts in the Candida genus are normally found in and on the human body, but in someone with a compromised immune system or other imbalances (sometimes caused by anti-biotics changing the balance of micro organisims), too much yeast can grow (known as Candidiasis or thrush) causing health problems.