Fiji is an image processing package. It can be described as a distribution of ImageJ together with Java, Java 3D and a lot of plugins organized into a coherent menu structure. Fiji compares to ImageJ as Ubuntu compares to Linux.
The main focus of Fiji is to assist research in life sciences.
For users, Fiji is easy to install and has an automatic update function, bundles a lot of plugins and offers comprehensive documentation.
For developers, Fiji is an open source project hosted in a Git version control repository, with access to the source code of all internals, libraries and plugins, and eases the development and scripting of plugins.
Custom Plugins can be found in the Analysis Software Repository page.