What is Adobe AEM?

CQ5 is a java based content management system from adobe, previously Day CQ5.
1)    It is based on a content repository(i.e it uses a content repository to store the content of a website) and use JCR(java content repository) specification to access the content repository.
2)    It uses RESTful Apache Sling framework to map request url to the corresponding node in content repository
3)    It uses powerful OSGi framework internally to allow modular application development. It means  individual pieces of your application(called bundles in terms of OSGi) can be independently started and stopped. CQ5 uses Apache Felix as the OSGi container. Therefore different parts of cq5 can be independently started and stopped.

Coming to why a content management system is required? Some websites are very dynamic in nature, content needs to be updated frequently, so it is easier to manage the content of such websites using a CMS.

Adobe CQ5 platform allows you to build compelling content-centric applications that combine Web Content Management, Workflow Management, Digital Asset Management and Social Collaboration. The product has been completely redesigned from Communiqué 4, allowing Adobe to use new architecture and technologies, thus increasing functionality while reducing complexity. Extensive use of standards helps ensure long-term stability.

Adobe CQ5 is geared specifically for large companies and corporations with substantial -and often global- infrastructures. It combines web content management, digital asset management, and social collaboration to deliver a solution that allows large companies to manage incredible amounts of information, multiple internal and external websites, a myriad of media assets, and detailed workflow