A singleton is an object class designed such that only one instance of that class will ever exist. This can be useful to minimize resources required (e.g., database connections) or ensure consistent data throughout an application (e.g., user preferences).
I often tell my students that I am lazy as a programmer. I typically try to convince them that this is a good thing. The point I'm trying to make, of course, is that often the best line of code is the one you don't have to write.
A lot of people thought I was crazy for wanting to develop my own PHP framework, and 1400 hours of discovery and development later, they may have a point. At the end of the day, though, I have an extremely powerful and flexible framework for making database-driven web applications. Where even I drew the line, however, was with javascript. I made the decision to use jQuery because its design philosophy seems similar to my own.