Zach Beane points to a nice post by Nicolas Hafner on Generic Functions and CLOS. CLOS is often considered a hideously complicated system but the basics are easy to understand and use and most people will never need to explore the complex parts. Hafner’s post reminds me of Joe Marshall’s excellent Warp Speed Introduction to CLOS.
Together, these two posts serve as a sort of orientation to the Common Lisp object system. Both approach CLOS through generic functions, which are useful whether or not you ever define or instantiate an “object.” Both are short and definitely worth a read.