Containers are used to gain access to services. The followings are important characteristics of services:
Because of the Dependency Injection concept, the interface and implementation for a service are usually separate.
Each service has to be implemented as a singleton, which means it is created only once per portal container in a single instance.
A component equals a service. A service must not be a large application. A service can be a little component that reads or transforms a document where the term "component" is often used instead of service.
For example, in the /lib folder, you can find services for the following databases, caching, and LDAP:
exo.core.component.database-x.y.z.jar
exo.kernel.component.cache-x.y.z.jar
exo.core.component.organization.ldap-x.y.z.jar