There are 12 known B vitamins. Each B vitamin has a unique use within the body. B1, also known as thiamine, is just one of the types of B vitamins.
When choosing a supplement, you can either choose a thiamine or a B complex supplement. B complex vitamins are a blend of B vitamins. They may not contain all B vitamins, though, so it's important to read the label to learn which B vitamins are included in the blend.