1.1. What is Cancer?
Cancer is the diverse group of diseases, which is described by unstrained cell growth. Classification of cancers is based on the cell types and organs in which they first appeared. Since this type of growth can usually occur in all parts of the body so cancer and cancer development has 100 different types based on this malignant growth. Cancer is highly complex and heterogeneous disease but, at the same time, all malignant growths show some identical features. Those features are named as cancer hallmarks that are developed during cancer development .
The primarily suggested hallmarks are independent growth in the absence of growth signal, no response to growth inhibitors, escape of apoptosis, infinite replication ability, nourished development of blood vessels, metastasis and invasion of tissues . There is a further increase in the hallmarks of cancer like escape from immunity and deregulation of cellular energy metabolisms  Furthermore, additional characteristics are inflammation and instability of genome due to mutations . Usually there are two types of tumors such as benign tumor and metastatic tumor occurred during cancer development.
Cancer is usually sketched as the disease of genome because it gained the characteristics of cancer by mutation and instability of the genome. At the same time it is assessed that about 5-10% of cancer occur due to the genes and rest of the 90-95% occur dues to the environmental factors (Figure 1).
Around the World, Bacterial, parasite and viral infections caused 18% of the cancers. Most of these infections which lead to cancer are caused by oncogenic viruses. In humans the percentage of cancers caused by Human Papillomaviruses is 5.2%, 4.9%, by Hepatitis C & B viruses, 1% by Epstein Barr virus, 8-0.9% by HIV and Human herpes Virus. These viruses affect different crucial pathways of cell and interfere with growth, replication, apoptosis, uncontrolled cell division, progression through cell cycle and stability in genome. Thus, the affected cells gained the characteristics of cancer cells, which result in the formation of tumor .
Development of cancer is a multistep process which involves mutation and selection of cells due to the increase in growth, proliferation, invasion, survival and metastasis of cells. The first step in the development of cancer is the initiation of tumor due to the genetic mutation which lead to the abnormal division of a single cell. Uncontrolled proliferation results in the formation of tumor. Progression of tumor continues as additional alterations in genome of cells occur in the population of the tumor cells. These mutations offer different selective advantages to the cells like indefinite rapid growth and cell having these types of mutation will become dominant in the population of tumor, this is called clonal selection as a new population of tumor cells has developed having increased growth, survival, invasion which is a selective advantage. Tumors become more malignant and rapid growing duo to the clonal selection of the cells  (Figure 2).
Cancers are basically classified into six types on the basis of types of tissues which are following:
Cancer may be classified by the site in which they primarily originate. So, these may be of different specific types like lung cancer, liver cancer, brain cancer, and breast cancer etc.
Mixed types contain components in two different categories. Some of the examples are adenosquamous carcinoma, mixed mesodermal tumor, carcinosarcoma and teratocarcinoma.