What is Microsoft office 2003?

It is an application software.

Microsoft Office is a suite of productivity programs.
Baically you have a word processor, shreadsheet, e-mail client, database and presentation maker.
Further information can be found here: Microsoft Office 2003

You know, you could have just typed that into Google!
It's a sophisticated piece of software and not at all easy to learn (Word 1.0 had about 100 tools & options compared to it's latest version which has something like 2000! )

If you are looking for an Office program and you don't want to spend much, take a look at Open Office - it's free, and it's very good.