For the benefit of those who don't know what OSGi actually is, I wrote an article on my blog, which I am reproducing here, mainly because I see this question asked quite a lot. Most articles already assume that people know what OSGi is, but all I assume is that the reader is a Java programmer and familiar with the Eclipse IDE, in which case the reader has already been using OSGi without even realising it.
The Eclipse IDE is constructed from Plugins. Plugins are effectively OSGi bundles with a bit extra (usually a plugin.xml, sometimes a little more).
Eclipse is built on top of Equinox which is an implementation of the OSGi specification and then goes on to extend it with the Plugin Registry.
At its core an OSGi bundle is a standard Java JAR file with some extra information in the MANIFEST.MF which describes the bundle to the OSGi platform it is deployed to. The manifest describes things like the bundle's name, version, what packages and/or other bundles it depends on, what packages it exposes, and probably a fair chunk of other stuff I haven't mentioned.
OSGi is a service oriented architecture. This is quite an important point to me as most people hear or read SOA and automatically assume Web Services. Well Web Services are just one type of SOA. The specification of OSGi is inherently, and by design, service oriented.
Like I said, bundles can declare what packages they expose, but more importantly they declare and/or register services.
A service is essentially a Java interface and can be implemented by one or more bundles. A bundle that needs to use a particular service looks it up (or "tracks" when services are registered/unregistered) and then decides which service to use either by taking the first one it finds, or by homing in on a service that meets a specific need by specifying some service properties.
The most important thing is that services are dynamic. They can come and go and this is a part of the specification and something that any one who writes bundles has to accept and deal with.
As a result, OSGi has a tightly defined classloader mechanism which manages version control. You can have two bundles installed on your OSGi platform that expose the different versions of the same package, then other bundles can be explicit about which package version they depend on or not.
The OSGi specification consists of a core specification and a service compendium. The core specification is enough to write modular Java applications and the service compendium details a selection of services that OSGi implementations will usually implement a common set of, if not all of. These include things like Logging, Declarative Services (services can be declared using XML), HTTP service (you can expose web resources from a compliant container, some even support JSPs!) and others. See this page for more information:
Finally, what I haven't mentioned is that OSGi has been around for nearly a decade. It was originally intended for use as a dynamic platform for embedded devices but IMHO is really starting to make its mark through use on the desktop and (enterprise) server areas, especially in the last couple of years.
When you get in to it, the power of OSGi is mind boggling. Yet OSGi is simple and logical, while remaining comprehensive. OSGi is the dynamism and modularity that Java is currently missing.
I hope my brief description of OSGi does it justice. I deliberately wanted to keep it short and sweet and whet some appetites.
This tutorial from the Knoplerfish team will introduce you to the Knoplerflish OSGi implementation, get you building a bundle, creating and consuming services. It is also a good idea to then try out your bundle on other implementations such as Equinox, to give you a flavour for how different the implementations can be, while being able to run the same code.