A build tool (like Ant) allows you to "build" your applications.
Using xml you tell it what to do - which files to compile, where to put them, etc. etc.
You can set up different "tasks" -
ie. 'clean' which could clean out your build directories,
'build' which could compile your code,
'deploy' which could copy your code, etc.
You can even make tasks dependant on other tasks, so you could have a 'full deploy' task that first invokes 'clean', 'build' and then 'deploy'.
Take a look here also :
http://en.wikipedia.org/wiki/Apache_Ant MG
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