The Latin root words ultra and infra mean above and below respectively. Ultraviolet and infrared are scientific terms for electromagnetic radiation just beyond the range of frequencies that human eyes can see. Ultraviolet rays are those whose frequencies are slightly above that of violet light, while infrared rays have frequencies a little below that of red light. Think back to your school days: the colours of the visible spectrum are red, orange, yellow, green, blue and violet, in order of increasing frequency.
In electromagnetic radiation, frequency and wavelength are inversely proportional. Therefore waves can be specifically described by wavelength just as well as by frequency. Often wavelength is easier to measure, so people tend to use wavelength in describing radiation. But they still use the terms ultraviolet and infrared as I said above, thus inverting the natural meanings of ultra and infra.