can you list some services which you can customize using hooks?

Using Liferay's dummy wrapper classes  we can override  the liferay's services in HOOKs plugins

• UserLocalService  : service for adding, removing, and retrieving user accounts

            the wrapper for UserLocalService is UserLocalServiceWrapper

• OrganizationLocalService: adds, deletes and retrieves organizations. Also assigns users to organizations and retrieves the list of organizations of a given user.
  
• GroupLocalService: adds, deletes and retrieves sites.
  
• LayoutLocalService: adds, deletes, retrieves and manages pages of sites, organizations and users.