Source of infection is defined as the person, animal, object or substance from which an infectious agent passes or is disseminated to the host.
A reservoir is defined as any person, animal, arthropod, plant, soil, substance or combination of these in which an infectious agent lives and multiplies. The infectious agent depends on the reservoir for survival and it reproduces itself in such as manner that it can be transmitted to a susceptible host. In short, reservoir is the natural habitat in which the organism metabolizes and replicates.
Reservoir and source are not always synonymous. E.g. Hook worm infection - the reservoir is man but source of infection is infected host.