Barrel

A  Barrel is a block used to store fluids. There are 3 types of barrels: wooden, steel and lead, and they are progressively able to store more types of fluids. There is also a "dewar" which is capable of storing everything a steel barrel can store, as well as fluids that need to hold onto a very low or high temperature, such as liquid nitrogen.

Fluid cables can be connected to the top side of a barrel.

A capsule can be placed in the top slot of a barrel GUI to transfer fluid into the barrel, or in the bottom slot, to transfer fluid from the barrel into the capsule.

A filler, which is a creative-only item that exists for each type of fluid, can be placed in the top slot to keep the barrel permanently filled with a specific fluid, regardless of how much fluid is pulled out (making an infinite supply).

Crafting
Wooden barrel, capable of storing water:

Steel barrel capable of also storing lava and natural gas:

Lead barrel, capable of also storing refined uranium and plutonium: