G proteins, short for guanine nucleotide-binding proteins, are a family of proteins involved in second messenger cascades.
G proteins are so called because they function as "molecular switches". They alternate from 'inactive' guanosine diphosphate (GDP) to 'active' guanosine triphosphate (GTP), which is a binding state, and which proceeds to regulate downstream cell processes.
Source: [Wikipedia](https://en.wikipedia.org/wiki/G_protein)