An 
organ is a distinct region of 
cells within a multicelluar organism that perform a distinct function. Organs include the 
heart, 
lungs, 
brain, 
stomach, 
spleen, 
pancreas, 
kidneys, 
liver, 
intestines, 
uterus, 
bladder etc.  Some considered 
skin as the largest organ on the human body.
Analogous structures exist on the sub-cellular level and are known as organelles.
See also : transplant, Artificial organ
All Wikipedia text 
is available under the 
terms of the GNU Free Documentation License