t-channel
Two particles scatter by exchanging a virtual particle. The exchanged particle carries momentum transfer $k^2 = t = (p_1 - p_1')^2$, which is spacelike for elastic scattering. Dominates at small scattering angles.
Structure
Each incoming particle continues as an outgoing particle, connected by a single internal propagator running between the two fermion lines. The upper line is unchanged in identity; the lower line likewise.
Where it appears
Coulomb scattering (one-photon exchange), Moller scattering t-channel, Bhabha scattering exchange channel. The familiar $1/t$ pole gives the forward scattering peak.