One way of thinking about Seam is that it unifies JSF and EJB/JPA. This way you app does not have to deal with two disparate models, one for the user interface (JSF) and the other for business methods (EJB) and persistence (JPA).
As Peter said - Seam is basically an integration framework that bringd JSF and EJB together.
Assuming you are aware of Struts/JSF. The SEam framework allows you to use EJB3 as action classes/managed beans.
Since you are able to use EJB3, you get the benefits like transaction management, security and all the goodies.
Secondly Seam provides new context scopes like conversation and business. The new JBoss EL is also good.
All these combine to remove much of the plumbing code required to use JSF, Hibernate/JPA etc.
Add Richfaces/Icefaces on top of this to give a sleek interface and AJAX.