The Gs alpha subunit (Gαs, Gsα, or Gs protein) is a heterotrimeric G protein subunit that activates the cAMP-dependent pathway by activating adenylyl cyclase. A mnemonic for remembering this subunit is to look at the first letters in each word (Gαs = Adenylate Syclase).
It is one of the three main families of G proteins: Gαi/Gαo, Gαq, and Gαs.
The G protein-coupled receptors that couple to this kind of G-protein include:
The general function of Gs is to activate adenylyl cyclase, which, in turn, produces cAMP, which, in turn activates cAMP-dependent protein kinase. Further effects of Gs are thus found in function of cAMP-dependent protein kinase.
It provides a step in signal transduction. Amplification of the signal occurs for instance because the receptor activates several Gs.  However, each Gs activates only one adenylate cyclase.