One of their most distinctive features is their radial symmetry which also tells us about how they function. There is no complex nervous system, only a nerve net, because cnidarians do not consistently orient in any particular direction. If you did always face the same direction then there would be obvious advantages to concentrating sensory structures to sense what is ahead. Cnidarians don't do that and instead sensory structures are evenly spaced around the whole organism.
Cnidarians are also diploblastic , with only ectoderm and endoderm as tissues.
Unlike the first experiments with multicellularity which resulted in the Porifera cnidarians are organised at the tissue grade rather than the cellular grade . Diploblasty and radial symmetry seem to have occurred at the same time during the evolution of the different body plans and there are no examples of diploblastic bilaterally symmetric animals, nor are there radially symmetric triploblasts . Other features of importance to understanding the Cnidaria are the unique cnidocytes , or stinging cells, that give the phylum its name. Cnidarians are predators and use a amazing array of different nematocysts to puncture, jab, stick to, and entrap their prey. The Cnidarian life cycle is also unique to the group with an alternation of generations between the medusa and the polyp stages.