EL84s are power amp valves commonly used in British styled amps. They're a noval power valve of approximately 15 Watts of output. The EL84 is a pentode output valve, meaning they have an additional grid to increase the output of the valve. They function in a different way to a beam tetrode (like 6V6s for instance), giving them an different tonal structure.
EL84s are commonly considered to be middy and chimey, with a glassy top end. They are commonly found cathode biased in many low wattage vintage amps for a beautiful touch sensitive compressed character.