Let's look at Sun's
J2EE Bible: Designing Enterprise Applications with the J2EE Platform, Second Edition, at page 253:
8.1.2 Transaction Participants
An application that uses transactions is called a transactional application. In a J2EE application, a transactional application may consist of multiple
servlets,
JSP pages, and enterprise beans. A resource manager is an external system accessed by an application. A resource manager provides and enforces the ACID transaction properties for specific data and operations. Examples of resource managers include a relational database (which support persistent storage of relational data), an EIS
system (managing transactional, external functionality and data), and the
Java Message Service (JMS) provider (which manages transactional message delivery). A transactional application accesses a resource manager through a transactional resource object. For example, a
JDBC java.sql.Connection object is used to access a relational database. A resource adapter is a system library that makes the API of a resource manager available to an application server. A Connector is a resource adapter that has an API conforming to the Java Connector architecture, the standard architecture for integrating J2EE applications with EISes.