Classes vs. Functions

Could someone please explain the difference between classes and functions and how to use a class. I write alot of PHP, but I never understood this at all. I use an include statement in many of my pages and include a file with a bunch of functions. For instance, I might have a function called stock(); In the page I am using I include the file that has this function and I call it like this: stock($Sym); I am wondering if I am doing it the wrong way. So I need to better understand classes. What is one, and why would you use it? Thanks.