JCA: Java Connector Architecture: API used in integration efforts with legacy systems.

JDBC: JDBC 2.0 Option Package is a J2EE API for connecting to databases and executing SQL commands. It provides the abstraction that shields developers from needing a low-level knowledge of their database’s APIs and drivers. There are a number of different methods for using JDBC to interact with databases including direct access using their native drivers or via an access server. Most often used for connecting to databases, pooling multiple connections to databases, managing distributed transactions, etc. JDBC Core API is a smaller API used in J2SE.

JFC: Java Foundation Classes. These classes are used to create desktop and web applications and includes GUI components.

JMS: Java Message Service, APIs used for asynchronous communications. The API handles message queuing and point-to-point and publish /subscribe types of message-oriented middleware services.

Jini: Jini is not an acronym and as far as I know doesn’t stand for anything. Drop me a line if you know the history of the name. I added it in here because it seems like it should stand for something like “Java InterNetwork Interface” or something similar. Jini is a interface technology that allows for the quick and easy creation and destruction of connections between networked Java components. Jini is Considered by many to be the next step in the evolution of RMI.