What is aspect-oriented programming?

AOP is all about managing the common functionality (which span across the application, hence cross cutting) within the application such that it is not embedded within the business logic.

Examples to such cross cutting concerns are logging, managing security, transaction management etc.

Frameworks allows this to managed automatically with the help of some configuration files.

Class and method attributes in .NET are a form of aspect-oriented programming. You decorate your classes/methods with attributes. Behind the scenes this adds code to your class/method that performs the particular functions of the attribute. For example, marking a class serializable allows it to be serialized automatically for storage or transmission to another system. Other attributes might mark certain properties as non-serializable and these would be automatically omitted from the serialized object. Serialization is an aspect, implemented by other code in the system, and applied to your class by the application of a "configuration" attribute (decoration) .

Laymans terms so let me give you an example. Let's say you have a web application, and you need to add error logging / auditing. One implementation would be to go into every public method and add your try catch blocks etc...

Well Aspect oriented says hogwash with that, let me inject my method around your method so for example instead of calling YourClass.UpdateModel(), the system might call,

LoggingHandler.CallMethod() this method might then redirect the call to UpdateModel but wraps it in a try catch block to handle logging errors.

Now the trick is that this redirection happens automagically, through configuration or by applying attributes to methods.

This works for as you said cross cutting things which are very common programing elements that exist in every domain, such as: Logging, Auditing, Transaction Mgmt, Authorization.

The idea behind it is to remove all this common plumbing code out of your business / app tier so you can focus on solving the problem not worrying about logging this method call or that method call.