EL34s are power amp valves commonly used in British styled amps. They're an octal power valve of approximately 25 Watts of output. The EL34 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 6L6s for instance), giving them an different tonal structure.
EL34s are commonly considered to be middy and contribute to the brash, mid range bark of many of the common vintage British amplifiers from the 60s.