Primary brain tumors are classified by the type of tissue in which they arise. The most common brain tumors are gliomas, which begin in the glial (supportive) tissue. There are several types of gliomas, including the following: 1  Astrocytomas arise from small, star-shaped cells called astrocytes.rimary brain tumors are classified by the type of tissue in which they arise. The most common brain tumors are gliomas, which begin in the glial (supportive) tissue. There are several types of gliomas, including the following: 1  Astrocytomas arise from small, star-shaped cells called astrocytes.