RNA and DNA are related because they share three base molecules: adenine, guanine and cytosine. RNA also contains uracil, which DNA doesn't have.
DNA is found within chromosomes. When a cell divides, or replicates, the DNA is copied. RNA comes into the picture when it is time to copy the DNA and send it to the cytoplasm of a cell, where protein synthesis takes place. DNA can't copy itself without RNA present and several types of RNA are used to copy and transmit genetic information. A good example of how DNA and RNA work together is a newborn baby's development. Without the relationship between RNA and DNA, a single sperm and egg couldn't produce a baby which has five trillion cells, each of which carries the same DNA.