The first law of thermodynamics states that  energy can be changed from one form to another, but it cannot be created or destroyed. The total amount of energy and matter in the Universe remains constant, merely changing from one form to another. The First Law of Thermodynamics (Conservation) states that energy is always conserved, it cannot be created or destroyed. In essence, energy can be converted from one form into another. Work is the path way for the transfer of energy to one form to another.  Consider the shot putter, the work the shot putter does on the shot putter takes the chemical energy in the his muscles and converts it into kinetic (and eventually to potential energy).

When thinking of the first law of thermodynamics on a small scale we use the word "system".  A system is a group of objects that interact with other, the earth and the moon is a system, so is a book on a table, the book, the table and the earth are all part of a system.  When we consider Net work we look at out side sources acting on objects in the system.  Take the book, table, and Earth system, if the book is pushed of the table, the book falls to the Earth, the Earth does work on the book, the book also does work on the Earth, and it turns out to be the same amount on for (the system need to be conserved).  If we only consider the book then the Earth does work on the book and the motion of the book increases.