I can find the technical explanation of what data mining is in a book or on Wikipedia, but I'm wondering what sort of development does it exactly involve? Is it more about using tools or more about writing tools? Is it really any much different from other domains when it comes to R&D?
Data Mining is the process of discovering interesting patterns in large amounts of data. It is not querying data, which is just what user Treb describes (sorry Treb).
To understand DM from a developer's perspective, you should read the book Programming Collective Intelligence by Toby Segaran.