Introduction
Jini technology is a technology for building Service-oriented architecture that defines a programming model which both exploits and extends Java technology to enable the construction of secure, distributed systems consisting of federations of well-behaved network services and clients. Jini technology can be used to build adaptive network systems that are scalable, evolvable and flexible as typically required in dynamic computing environments.
The term Jini refers to both a set of Jini_Specifications and an implementation; the latter is referred to as the Jini_Starter_Kit]. Both the specifications and the Starter Kit (initially created by Sun Microsystems) have been released under the Apache 2.0 license and have been offered to the Apache Software Foundation. It is now available as an Apache top level project called River.
Once you've understood Jini's core concepts, move on to Getting Started.
Concepts
The Jini Architecture Specification page gives an excellent overview of Jini. Here are some additional topics that may be of interest: