Carcinoma is a malignancy that develops from epithelial cells. Specifically, a carcinoma is a cancer that begins in a tissue that lines the inner or outer surfaces of the body, and that arises from cells originating in the endodermal, mesodermal or ectodermal germ layer during embryogenesis.Carcinomas occur when the DNA of a cell is damaged or altered and the cell begins to grow uncontrollably and become malignant. It is from the Greek: καρκίνωμα, romanized: karkinoma, lit. sore, ulcer, cancer (itself derived from karkinos meaning crab). Classification
As of 2004, no simple and comprehensive classification system has been devised and accepted within the scientific community. Traditionally, however, malignancies have generally been classified into various types using a combination of criteria, including:The cell type from which they start; specifically:

Epithelial cells ⇨ carcinoma
Non-hematopoietic mesenchymal cells ⇨ sarcoma
Hematopoietic cells
Bone marrow-derived cells that normally mature in the bloodstream ⇨ leukemia
Bone marrow-derived cells that normally mature in the lymphatic system ⇨ lymphoma
Germ cells ⇨ germinomaOther criteria that play a role include:

The degree to which the malignant cells resemble their normal, untransformed counterparts
The appearance of the local tissue and stromal architecture
The anatomical location from which tumors arise
Genetic, epigenetic, and molecular features

Histological types
Adenocarcinoma
(adeno = gland) Refers to a carcinoma featuring microscopic glandular-related tissue cytology, tissue architecture, and/or gland-related molecular products, e.g., mucin. Squamous cell carcinoma
Refers to a carcinoma with observable features and characteristics indicative of squamous differentiation (intercellular bridges, keratinization, squamous pearls).
Carcinoma of unknown primary site
The term carcinoma has also come to encompass malignant tumors composed of transformed cells whose origin or developmental lineage is unknown (see cancer of unknown primary origin; CUP), but that possess certain specific molecular, cellular, and histological characteristics typical of epithelial cells. This may include the production of one or more forms of cytokeratin or other intermediate filaments, intercellular bridge structures, keratin pearls, and/or tissue architectural motifs such as stratification or pseudo-stratification. ICD-10 code
(8010-8045) Epithelial neoplasms, NOS
(8050-8080) Squamous cell neoplasms
(M8070/3) Squamous cell carcinoma, NOS
(8090-8110) Basal cell neoplasms
(M8090/3) Basal cell carcinoma, NOS
(8120-8130) Transitional cell carcinomas
(8140-8380) Adenocarcinomas
(M8140/3) Adenocarcinoma, NOS
(M8142/3) Linitis plastica
(M8155/3) Vipoma
(M8160/3) Cholangiocarcinoma
(M8170/3) Hepatocellular carcinoma, NOS
(M8200/3) Adenoid cystic carcinoma
(M8312/3) Renal cell carcinoma
(M8312/3) Grawitz tumor
(8390-8420) Adnexal and Skin appendage Neoplasms
(8430-8439) Mucoepidermoid Neoplasms
(8440-8490) Cystic, Mucinous and Serous Neoplasms
(8500-8540) Ductal, Lobular and Medullary Neoplasms
(8550-8559) Acinar cell neoplasms
(8560-8580) Complex epithelial neoplasms

Carcinoma In situ
The term carcinoma in situ (or CIS) is a term for cells that are significantly abnormal but not cancer. They are thus not typically carcinomas. Pathogenesis
Cancer occurs when a single progenitor cell accumulates mutations and other changes in the DNA, histones, and other biochemical compounds that make up the cells genome. The cell genome controls the structure of the cells biochemical components, the biochemical reactions that occur within the cell, and the biological interactions of that cell with other cells.