Seam is next generation web framework that integrates JEE technologies with variety of non standard technologies into a unified, consistent, programing model. Although Seam is not much popular in the co-operate world today It will be the next generation integration framework in Java world. The idea of seam is to glue the two key components of Java EE 5 specification that is JSF 1.2 and EJB3 (JSF is the standard presentation framework for the web tire that provides both a UI component model and a server side event model. EJB3 is the standard programing model for creating secure and scalable business components that access transactional resources. EJB3 also surround JPA that defines a standard persistence model for laying a ORM layer in between application and RDMS).
In conventional JEE programing model it requires lot of ''glue" code (backing bean layer) to combine JSF and EJB layers. Simply Seam eliminates that additional layer and fits JSF and EJB together. As such and with many more programing patterns, Seam has positioned as the next generation integration framework for Java base web applications.
Seam project is an idea of the Hebernate's inventor Gavin King. As for the most software project, Seam also was started to scratch a single developer's itch that of Gavin's. Why? Gavig King was fed up with that developers are not using Hibernate properly with the stateless framework like Spring. So he identified that the missing integration between JSF and EJB in a statefule context is much needed and it would avoid further abuse of Hibernate as a JPA provider. This would allow the persistence context (Hibernate Session or JPA EntityManager) to transcend layers and would permit stateful session beans to respond JSF directly. To achieve this, Seam was came out as a stateful, yet efficient, architecture in to the JEE picture.
Even there are many features that I can write on this blog I would not because there are lot more well written article that describes in and out about Seam well. If you are further interested it is just a matter of searching(Googling).