<p>What role is Spring taking in Struts + Spring + Hibernate?</p>

Spring provides many different "modules" and different programmers will use different parts of Spring.
However, commonly in this sort of stack, you will see Spring being used as a provider of
An inversion of control container for dependency injection
An abstraction to Hibernate called "HibernateTemplate"
Framework classes for simplifying Aspect Oriented Programming
Transaction support, often "declaratively" via the IoC container and AOP.