Threonine (THR or T) is a hydrophilic amino acid. Its shape in Foldit is identical to that of valine. It's also similar in shape to asparagine or aspartate. The CPK or EnzDes coloring options (found in view options) can be used to help distinguish these amino acids.
Threonine'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 Threonine on wikipedia.