Energy- a conserved, substance-like quantity with the capability to produce change.

The idea of energy is an invention that proves very useful. Energy can be moved around and stored in a variety of ways, but the energy itself is unchanged. Energy is universal and it does not come in different "kinds" or exist in different "forms." There are many mechanisms for energy storage such as elastic E_e, kinetic E_k, gravitational potential E_g,  and chemical potential E_chem, where the energy can be easily retrieved. Also, energy can be stored in the random motion of molecules E_thermal or the wave motion of molecules E_sonic where the stored energy is very difficult to retrieve. Friction within a system often causes energy to be unretrievably stored in E_thermal and E_sonic which we lump together as energy dissipated, E_diss.
 
A numerical amount of energy can be calculated for each storage mechanism.  As energy is transferred from one method of storage to another, the total amount of energy stays constant (energy is conserved). When examining energy transfers, it is helpful to choose what methods of energy storage are in our system and what methods are outside our system. Generally, the smallest system that contains all the needed ways of storing energy is the easiest. Transferring energy from one storage method to another or transferring energy into or out of a system is the process of "working" (your textbook simply calls it "work.")

Money analogy: We will define "the system" as the personal and institutional places where you keep your money. You can store your money in a number of ways, in a checking account, savings account, cash in a piggy bank, or a stock mutual fund. As you transfer money from cash and savings to checking, the amount of money stays the same (is conserved) even though the money is now stored somewhere else. Some transfers cost you money, such as using a debit card or getting a cash advance. In this case the money is transferred out of your account and into the bank's account. The money still exists, you just can't have it anymore. (This is like dissipated energy.)

Bar graphs and energy flow diagrams are useful for quantitative analysis of the energy flow during a change. The initial stored energy of a system is represented with a bar for each storage mechanism.  To the right of the bar graph, an energy flow diagram indicates which items are inside and outside of the system and whether or not energy is transferred in or out of the system by working or heating.  Finally, to the right of the energy flow diagram, another bar graph indicates the final distribution of energy stored within the system. To use the representation for quantitative analysis, an mathematical expression or energy value can be written below each bar and the energy flow diagram. The sum of the initial stored energy in the system plus or minus any energy flowing in or out of the system equals the sum of the final stored energy in the system.