Energy
Energy is often found in Mods in relation to Power and Machinery. It is implemented in different ways across Mods and Mod Loaders.
Minecraft Joules (MJ)
Back many years ago, the mod BuildCraft had an Energy system known as Minecraft Joules (MJ). Not Megajoules, as you'd expect.
MJ could be generated by using BuildCraft's Redstone Engine, Stirling Engine or Combustion Engine.
However, other Mods added their own forms of MJ generation, like Forestry with its Clockwork Engine.
MJ was used by Blocks like BuildCraft's Quarry, Filler or Mining Well, and Blocks from other mods.
MJ could be transported by using BuildCraft's Kinesis (aka Conductive) Pipes. There are nine different variants of this Pipe, some with different transfer rates, or extra functionality: ftbwiki.org/Kinesis_Pipe
Redstone Flux (RF)
Eventually, a standard Energy system was created by CoFH, known as Redstone Flux (RF). This was used by almost any Mod with Energy capabilities.
Some Mods with different energy sources, such as Immersive Engineering, would still use their own systems alongside RF for a while, and just convert the two types when transferring in / out of Energy blocks.
Forge Energy (FE)
During 1.12, the LexForge Mod Loader made its own Energy system called Forge Energy (FE). It became the main standard for 1.12 and later for Mods on LexForge.
Energy Units (EU)
Some Mods used a different form of energy called Energy Units (EU), notably Industrial Craft (IC) and Greg Tech.
EU could be generated by various IC Generator blocks, and was used by most of its Machine blocks.
EU could be transported by using Cables.
EU has different Voltages, Low, Medium and High (LV, MV, HV)*. Higher voltages transfer more EU per tick.
Supplying a higher voltage than a Machine block supports will cause it to explode.
* GregTech added higher levels of Voltage, notably Extreme, Insane, Ludicrous and more.
Other less notable Energy systems:
Crystal Flux (CF)
CF is just like Redstone Flux (RF), made by Actually Additions. The two Energy types are not interchangeable, so RF cannot be used by Actually Additions blocks, and CF cannot be used by RF blocks.
Joules (J)
J is an Energy system made by Mekanism.
Any machine that requires J can also use Redstone Flux (RF) or Forge Energy (FE).
Micro Infinities (μl)
μl is like Redstone Flux (RF), made by Ender IO.
Any machine that requires µl can also use Redstone Flux (RF) or Forge Energy (FE).