Eclipse is a multi-language software development environment comprising an integrated development environment (IDE) and an extensible plug-in system. It is written mostly in Java. It can be used to develop applications in Java and, by means of various plug-ins, other programming languages including Ada, C, C++, COBOL, Haskell, Perl, PHP, Python, R, Ruby (including Ruby on Rails framework), Scala, Clojure, Groovy and Scheme. It can also be used to develop packages for the software Mathematica. Development environments include the Eclipse Java development tools (JDT) for Java, Eclipse CDT for C/C++, and Eclipse PDT for PHP, among others.
Eclipse is one of the most commonly used IDEs. It is a platform with loads of plugins. Be sure that you'll spent a lot of time configuring it. There is no way to use it effectively without understaing lots of setting options. Nothing is simple. However it is worth learning. As a developer you'd probably use it 8 hours a day so spending several hours on customisation might pay off. One yo've learned it you'd be able to create software very quickly and effectively. For me it is the best free IDE, but definetlny not the simplest. It is much better now with Eclipse Market.
Use it with great success for last 12 years