BUILD_NUMBER

The current build number, such as "153"

BUILD_ID

The current build id, such as "2005-08-22_23-59-59" (YYYY-MM-DD_hh-mm-ss)

JOB_NAME

Name of the project of this build, such as "foo"

BUILD_TAG

String of "hudson-${JOBNAME}-${BUILD_NUMBER}". Convenient to put into a resource file, a jar file, etc for easier identification.

EXECUTOR_NUMBER

The unique number that identifies the current executor (among executors of the same machine) that's carrying out this build. This is the number you see in the "build executor status", except that the number starts from 0, not 1.

To understand how environmental variables provided by Hudson can be utilized by Ant, study the following target:

<target name="printinfo">
  <property environment="env" />
  <echo message="${env.BUILD_TAG}"/>
</target>