What is OSGi and what are some examples of its use?

I've just started hearing the term OSGi being used (while reading tutorials on common Java EE containers such as GlassFish and Spring), however I have been unable to find a simple, straight-forward, easy-to-understand explanation of what OSGi is that an enterprise novice such as myself would understand.

Can someone provide such a dummy-proof explanation? Maybe with some examples or even code excerpts?

Thanks!

Simply said, OSGi is a dynamic module system for Java. It defines means to install, uninstall, update, start and stop modules. Those modules are called bundles, but are, in their simplest form, actually Java jar files with a special Manifest. Modules can be installed, uninstalled etc without stopping or restarting the Java VM.

An OSGi framework manages the described lifecycle of and dependencies between the bundles in a secure way. A bundle needs to state which Java packages it exports and which it imports. The import and export statements can be annotated with version information, so that you even can have more than one version of the same package in the same Java VM.

The OSGi Alliance is the organization that specifies the OSGi framework and many accompanying services, e.g. for managing configuration data, device access, etc.

This is just a very basic overview. OSGi is much more. Please have a look at https://www.osgi.org/resources/architecture/ (an introduction to OSGi's architecture) and https://www.osgi.org/resources/where-to-start/ (a lot of links and further readings recommended by the OSGi Alliance).

Update 07/2021
As of 2021 the OSGi Alliance moved all resources and work to the Eclipse Foundation. It continuous its work in the newly established OSGi Working Group.

At the expense of some down votes ;-), I'd like to answer this in my own words (so that I can get corrected on my understanding).

We build applications as a set of modules, each module functionally cohesive with-in itself and loosely coupled with other modules. This has many advantages as you might already know. Overtime time, the unit of modularity has improved from functions to classes to packages to a unit of deployment (like jar in Java and assembly in .Net). But all of this is only during development time, once the application (which is a set of modules) is deployed, the server still see's it as one giant monolithic application i.e. the logical boundaries are not preserved during runtime. OSGi makes these boundaries explicit during development as well as runtime apart from other benefits outlined here

I suggest this excellent free book to get you started OSGi in Practice

If you work on Java platform, also check this presentation: Why Have The OSGi Specifications Been Based On Java™ Technology?