EU - Energy Unit is the measure of energy used by IC. It is most similar to the SI derived unit Joule. EUs are 'produced' by generators, stored by mobile units like an RE Battery or by stationary units like a batbox, transmitted along cables, and 'consumed' by ICs various machines.
EU stored in items or devices does not leak over time.
EU is not related to Redstone current, but multiple machines can be affected by redstone currents, and a Detector Cable produces redstone current if there is any EU passing through it. Some machines allow a lever to be mounted directly on its block.
Raw EU values are used to measure capacity of energy storage items or blocks.
EU/t means Energy unit per tick. It is a derived unit of energy over time, a rate of EU consumption or production, similar to the SI derived unit Watt.
To measure the average EU/t transferred through a wire over time an EC-Meter can be used. If clicked once on a cable it will begin measurement. Clicking the same cable again will display the averagely measured EU/t and over how many ticks the measurement took place. It will also begin a new measurement on the same cable. The EC-Meter is not implemented in the latest version of IC2, and will not do anything.
EU/t transfer can also be observed by the rate of change in EU capacity displayed by storage devices like the batbox.
Minecraft normally operates at about 20 ticks per second, unless you are experiencing server lag. Therefore, under no lag conditions, the rate of EU/sec is 20x EU/t.
Machines and cables will explode if they are exposed to too much EU/t. This differs from the old system, where there was a separate "voltage." In the current system, voltage and EU/t are the same thing.