It is not easy to explain in few words what Eclipse™ is. (Definitely, the Eclipse we are talking about is not an astronomical phenomenon. It is also not about Vampires.) This book is about the Software Eclipse that can be extended to do almost anything.
To give technical details of Eclipse, we can throw many jargons to the reader. OSGI, Platform, Workspace, SWT, Run-time, plug-ins, PDE, etc. The architecture of Eclipse. How things get together within Eclipse. How it internally works. Instead, we would take a practical approach. We would try to hide the internals of Eclipse. We would see a quick highlight of what Eclipse can do.
Technically, Eclipse is a plugin based framework. As it is plugin based, it can be extended to do a lot of diverse things. Eclipse is not just a Java/J2EE IDE. It is also an IDE for web Development , Python , perl, PHP, Ruby on Rails , etc.
Eclipse is also not only a programming IDE. It is also,
A chat client
A bit-torrent client
Azurus (now called Vuze) 
Many of the Eclipse RCP (Rich Client Platform) based products
Even for C/C++ Development, it has/had many variants. Basic Eclipse CDT, CodeWarror, Windriver, ARM DS-5, etc.
To summarize everything, Eclipse platform overview page  clearly says:
Eclipse is an open (IDE) platform for anything, and for nothing in particular..
We would like to say that Eclipse is for Nothing in particular, but everything in general.
|||(1, 2) Aptana® Studio 2 (http://www.aptana.org/)|
|||IBM Developer Works article on Eclipse RCP|
|||Building VUZE in Eclipse|
|||Wascana is no longer developed/maintained|