Classification/Types of reductive groups

Let $G$ be a reductive group over a field $k$. What actually does it mean to say that $G$ is of type $A_n, B_n,\dots,G_2,{}^2A_n, {}^3D_4,...$?

In case it helps, I know what the Dynkin diagrams of types $A_n, B_n,\dots,G_2$ are (but not those of types ${}^2A_n, {}^3D_4,...$). I also know how to draw the Dynkin diagram from a root system. In addition, I know how to get a root datum from $G$.

This is probably well-known, but I can't find where it is defined. Thanks a lot!