A leecher is similar to the actual leech that thrives on blood of the host animal. Though parasitic in nature, they re not voracious organisms.. When they get full , they stop sucking and detach from their host ..some would think its not fair but leeches have important roles in ecology and maintaining that symbiotic relationship in the environment. However, human leechers are far quite different from the real ones, why? Some leeches only take without giving back and many are just like that..now you have an idea what a real leecher is..someone who thrives on another for survival yet doesn't become beneficial to his human host..in short it can be summed up in one word PARASITISM!!! Now u know!