Down quarks have a -1/3 charge and are fermions, which means they have a 1/2 spin. The anti-down quark has a charge of +1/3 and they both have a strangeness of 0.
When this flavour of quarks is combined with up quarks, protons and neutrons can be formed. If one up quark and two down quarks are combined, a neutron is created. If a particle has two up quarks and one down quark, it is a proton.
The charges of atomic nuclei are determined by the charges of their quarks. For example, a proton has two up (+2/3) and one down (-1/3), leaving it with an overall charge of +1.
Up and down quarks are the least heavy of the quark flavours, meaning that after the process of particle decay, they can be formed from other flavours.