Cnidarians are radially symmetrical. This means that their gastrovascular cavity, tentacles, and mouth are aligned such that if you were to draw an imaginary line through the center of their body, from the top of their tentacles through the base of their body, you could then turn the animal about that axis and it would look roughly the same at each angle in the turn. Another way to look at this is that cnidarians are cylindrical and have a top and bottom but no left or right side.
There are several sub-types of radial symmetry that are sometimes defined depending on the finer structural details of an organism. For example, many jellyfish have four oral arms that extend below their body and their body structure can therefore be divided into four equal parts. This type of radial symmetry is referred to as tetramerism. Additionally, two groups of cnidarians, corals and sea anemones, exhibit six- or eight-fold symmetry. These types of symmetry are referred to as hexamerism and octamerism, respectively.
It should be noted that cnidarians are not the only animals to exhibit radial symmetry. The echinoderms also display radial symmetry. In the case of the echinoderms, they possess five-fold radial symmetry which is referred to as pentamerism.