Can somebody please tell me in simple words what exactly is ant build file and maven build file?
They are XML files that describe how to build and package software (usually Java software).
Ant files are used by the Ant tool, which provides more flexible, free-form builds. An Ant build file is much like a program in its own right (only encoded as verbose XML).
POM files are used by Maven, which has a different philosophy and imposes more structure. The Maven approach is more declarative than Ant. Writing a Maven build is less like writing a procedural program to build the software.
They're just like any other build files (NAnt, make, ...).
They describe the steps taken to produce some kind of artifact - an executable file, a test run, etc.
The build file will describe how to create a clean build environment, what to compile, where the dependencies are that you need to compile, where to put the executables, DLLs, and the like, any additional programs you need to execute (like running your testrunner on your project, or instrumenting your code so a coverage tool can analyse your test coverage).