It has been known for a long time that there are certain objects in space, which appear, when observed with small telescopes, as very blurry, self-luminous patches. These objects possess structures of different types. Often they are spherical in shape, often elliptical, and many of them have a spiral-like appearance, which is why they are occasionally called spiral nebulae. Thanks to the enormous angular resolution of modern giant telescopes, it was possible to determine that these nebulae lie outside the bounds of our own Milky Way. Images taken with the 100-inch telescope on Mt. Wilson reveal that these nebulae are stellar systems similar to that of our own Milky Way. By and large, the extragalactic nebulae are distributed uniformly over the sky and, as has been demonstrated, are also distributed uniformly in space. They occur as single individuals or group themselves to clusters. The following lines intend a brief summary of the more important characteristics and a description of the methods that made it possible to establish these characteristics.