C. elegans is a free-living, non-parasitic soil nematode that can be found throughout the world. It is transparent and about 1mm in size. "Worm" is its nickname. Researchers working with C. elegans call themselves "worm people" and form the "worm community" of research.
Why study C. elegans over other more advanced organisms? The two main reasons are commonalities with other organisms, and ease of operation.
Organisms like C. elegans are known as model organisms because they can be used to closely study and elucidate our understanding of basic biological phenomena that might not be easily researched in other organisms. C. elegans was first selected by Sydney Brenner for study for its properties as a model organism.
As you might guess, there are various model organisms used to study different biological phenomena. For instance, prokaryotic model organisms (e.g. E. coli) are useful for understanding other prokaryotic organisms (e.g. bacteria) that are often valuable in human disease research.
Acknowledging the completion of C. elegans cell lineage and accompanying research in cell death, the Nobel Prize in Physiology or Medicine 2002 was awarded to three C. elegans researchers that contributed to the "understanding of genetic regulation of organ development and programmed cell death" 1.