Ace was created because many different mods have to use the same types of basic functions. With ordinary mods, these basic functions would be written and included with each addon. With Ace, this basic functionality is packaged into a single library that all Ace mods use. What this means to non-programmers is that Ace mods use fewer system resources, because they all use a the standard Ace code, which is loaded once for all of the Ace mods you have. If you're using non-Ace mods, these basic functions are loaded seperately for each addon you have -- and the more addons you have, the more this adds up. (On the flip side of this, if you only use one Ace addon, you don't gain any benefits, but if you use all Ace addons, a little efficiency improvement in each can result in a big difference.) For the programmers in the audience, Ace means you can write your addon quickly, because you have immediate access to basic Ace functionality (you don't have to reinvent the wheel -- you only have to write code for what you want to do).
Want to get started with Ace? Read on for a list of some of the best Ace addons out there.