u-channel
Like the t-channel but with the outgoing particles crossed: the exchanged particle connects the first incoming to the second outgoing and vice versa. Carries momentum $k^2 = u = (p_1 - p_2')^2$.
Structure
The fermion lines cross between initial and final state. Required by Fermi statistics when initial or final particles are identical. Enters with a relative minus sign for identical fermions.
Where it appears
Moller scattering u-channel (exchange diagram), Compton scattering u-channel, pair annihilation crossed diagram.