JBoss AOP is a 100% Pure Java aspected oriented framework usuable in any programming environment or tightly integrated with our application server. Aspects allow you to more easily modularize your code base when regular object oriented programming just doesn't fit the bill. It can provide a cleaner separation from application logic and system code. It provides a great way to expose integration points into your software. Combined with JDK 1.5 Annotations, it also is a great way to expand the Java language in a clean pluggable way rather than using annotations solely for code generation. Check out our User Guide linked below as it described in detail why you'd want to use aspects.
JBoss AOP is not only a framework, but a prepackaged set of aspects that are applied via annotations, pointcut expressions, or dynamically at runtime. Some of these include caching, asynchronous communication, transactions, security, remoting, and many many more. Check out our WIKI for more detail.