The Bell states are specific quantum states of two qubits that represent the simplest (and maximal) examples of quantum entanglement.
\vert\Phi^{\pm}\rangle =\frac{\vert 00 \rangle \pm \vert 11 \rangle}{\sqrt{2}}
\vert\Psi^{\pm}\rangle =\frac{\vert 01 \rangle \pm \vert 10 \rangle}{\sqrt{2}}
For more information see the wikipedia page on Bell states.