E34Ls (not to be confused with EL34s) are power amp valves commonly used in British styled amps. The E34L is a slight variation on the EL34, which you're likely to be familiar with. They are interchangeable, with the E34L being slightly warmer - perfect for fattening up a shrill preamp that can often be found in many EL34 equipped amps.
They're an octal power valve of approximately 25 Watts of output. The E34L 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.