From the Ant Website:
Apache Ant is a Java-based build tool. In theory, it is kind of like Make, but without Make's wrinkles.
Why another build tool when there is already make, gnumake, nmake, jam, and others? Because all those tools have limitations that Ant's original author couldn't live with when developing software across multiple platforms. Make-like tools are inherently shell-based -- they evaluate a set of dependencies, then execute commands not unlike what you would issue in a shell. This means that you can easily extend these tools by using or writing any program for the OS that you are working on. However, this also means that you limit yourself to the OS, or at least the OS type such as Unix, that you are working on.
Ant is a very useful tool for building primarily Java-based projects, and a ton more. The build instructions are in XML and are pretty easy to pick up.