Ant is a widely used compiling tool for many distributed web application. Eclipse provides supports to the Ant tool in the development environment. In order to use Ant Editor in Eclipse, file association should be set first. Or you can just create a new file named "build.xml", the Ant Editor will be the default editor for that file.
There's a basic tutorial which introduces several concepts in Ant and Eclipse, such as ant configurations, ant view, etc. It's available in http://individual.utoronto.ca/kia/ .
Another article from onJava.com: Integrating Ant with Eclipse .
Link from Eclipse online help on this topic: Eclipse Help about Ant .
Sometimes you need to prepare the MANIFEST.MF file to deploy your programs, a tutorial about this is available in Packaging Programs in JAR Files .