Benign tumors have normal-looking cells. That is, they have small and regular shaped nuclei, small nuclear volume relative to the rest of the cellular volume, a relatively low number of dividing cells, normal and well-differentiated tissue that has a well-defined tumor boundary. However, malignant tumors generally have all or several of the following characteristics: large and pleomorphic (irregular-shaped) nuclei, large nuclear volume compared to the rest of the cellular volume, a high number of dividing cells, disorganized and anaplastic (poorly differentiated) tissue that has a poorly defined tumor boundary.
Depending on the number of malignant characteristics present, the American Joint Commission on Cancer has recommended that the tumor be given a grade using G0 through G4.
G1 Well-differentiated (Low-grade and less aggressive)
G2 Moderately well-differentiated (Intermediate-grade and moderately aggressive)
G3 Poorly differentiated (High-grade and moderately aggressive)
G4 Undifferentiated (High-grade and aggressive)
Alternatively, Roman numerals I through IV may be used. Low-grade tumors are assigned lower Roman numerals (e.g. grade I), indicating that the tumor is less aggressive. High-grade tumors are assigned higher Roman numerals (e.g. grade IV), indicating that the tumor is very aggressive, growing and spreading quickly.
I Well-differentiated (Low-grade and less aggressive)
II Moderately well-differentiated (Intermediate-grade and moderately aggressive)
III Poorly differentiated (High-grade and moderately aggressive)
IV Undifferentiated (High-grade and aggressive)

These are descriptions of tumor cells (abnormal cells) based on cytologic appearance. Neuroendocrine cells comprise the diffuse neuroendocrine system and it's tumors. The cytologic appearance of the neuroendocrine cells is its' nucleus and cytoplasmic neurosecretory granules. A Well Differentiated Neuroendocrine tumor indicates that the tumor originates in the neuroendocrine system and the cells, although abnormal, closely resemble the normal cells of the sight of origin. Small cell carcinoma is considered a poorly differentiated neuroendocrine carcinoma. A carcinoma is a malignant tumor that originates from epithelial cells (cells that lines and covers such as the skin, intestinal and uterine lining, etc). These tumors can be well or poorly differentiated. If poorly differentiated, this terminology indicates that the appearance of the cells cytologically is so far away from the cellular appearance of the normal cells of the sight of tumor origin. Therefore, you cannot identify the sight of origin by the appearance of these abnormal cells because their appearance is so bizarre. The cells of small cell carcinomas are just that --small. So are the cells of neuroendocrine tumors such as Carcinoid, Neuroblastomas, etc. The cells of poorly differentiated carcinomas can be spindle in form also and variable in size from small to large. Therefore, ancillary testing such as Molecular testing and special stains are indicated to determine the point of origin of the tumor. Treatment is entirely different for well differentiated neuroendocrine tumors (malignant potential is often uncertain in these tumors), small cell carcinomas, and poorly differentiated carcinomas. Therefore it is important to distinguish between them in regard to treatment and patient outcome.

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Note added at 1 day1 hr (2013-04-09 17:53:26 GMT)
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They wish to distinguish between well differentiated neuroendocrine tumors, small cell carcinoma (which is an aggressive neuroendocrine tumor) and poorly differentiated carcinoma for treatment purposes.