A fluid is a material that can flow easily and includes both liquids and gases. These materials often contain energy that can be harnessed as primary energy. These include the harnessing of primary energy known flows like:
A fluid can be divided into two categories: incompressible and compressible fluids. Generally, fluids in a liquid state, like water, are incompressible fluids because their density essentially stays constant when the pressure changes. Moreover, compressible fluids (most gases) change in density if there is a change in pressure, meaning they have the ability to be compressed—making these fluids vital for things like heat engines.
Heat engines take advantage of this change in volume from a change in pressure to spin turbines. This process allows humans to take advantage of the chemical energy in coal, oil, natural gas and biofuels. The same process allows people to generate electricity in nuclear power plants and geothermal energy.
When temperatures and pressures get high enough, an exotic form of matter forms that is hard to identify as either a liquid or a gas since they exhibit properties of both. This state is called a supercritical fluid and is used in supercritical coal plants and supercritical nuclear reactors.
Fluid mechanics is the branch of physics or engineering that studies how fluids move and act. This is key for having a deep understanding of the primary energy flows listed above. This discipline takes years of extensive study to understand, but a good place to start is the special fluid portal on hyperphysics.