Down quarks (sometimes written as a "d") are very small particles that help make up many larger particles, like protons and neutrons. Down quarks have a charge of -1/3. Since Down quarks are fermions (which means that no two of them can exist in the same space at the same time), they have a spin of 1/2. Down quarks are the second lightest quark, the first being up quarks.
The antiparticle of a Down quark is a Down antiquark, or simply an antidown.