When a molecule donates a proton, it is called the conjugate base of the original molecule. For example, NH3 is the conjugate base of NH4+. NH4+ and NH3 are a conjugate acid-base pair. One can be transformed into the other by undergoing a simple acid-base reaction.
The conjugate acid acts as an acid, with a strength inversely proportional to the strength of its conjugate base. For example, since NH3 is a weak base, NH4+ is a fairly strong acid.