Serine (SER or S) is a hydrophilic amino acid. Its shape in Foldit is identical to that of cysteine. The EnzDes and CPK coloring options (found in view options) can be used to tell serine and cysteine apart.
Serine's sidechain contains an oxygen atom which can act as a hydrogen bond acceptor, and as a hydrogen bond donor. See the Sidechain Bonding Gallery and the Sidechain Bonding Table for other sidechain donors and acceptors.
For more, see Serine on wikipedia.