Calculus is the mathematical study of change, and real-life things that change 
are modeled by functions.  Precalculus is essentially the study of 
functions, with a few other related topics that supplement the study of 
functions and prepare students for calculus.  In this text we'll concern 
ourselves first with learning about general functions, and later with certain 
types of common functions with special properties, like 
polynomial, 
exponential, 
logarithmic, and 
trigonometric functions.  First, 
it is of critical importance to understand exactly what a function is.  In the 
following lessons, we'll discuss what makes a function a function, some general 
properties of functions, and a few basic categories of functions.  In this text 
we'll assume a general knowledge of algebraic principles of solving equations, 
working with the real numbers, and working with sets.  In the last of the 
upcoming sections, we'll learn how functions behave under operations like 
addition, subtraction, etc.