Gigapixel images are pictures made up of at least 1.000.000.000 pixels. As there is currently (2011) no single camera commercially available (at least at a reasonable prices) with that kind of resolution, gigapixel images are usually made up of a series of overlapping photos, which are then stitched together creating a new image with this superior resolution. The benefits are that you capture such a huge amount of detail that you may discover things on the resulting picture which you may not have been able to see with your bare eye from where the gigapixel had been taken. Gigapixel images are mostly taken of skylines and landscapes as there is a lot of detail to discover. However, they are also used in architecture and art, both to conserve and to explore details of e.g. buildings and paintings.
Creating a gigapixel photo is a time consuming process, despite the fact that automated tripod heads have made it a lot easier recently. Stitching however still takes time and manual editing of the final result too. Here is an example:
Above you can see a 5.7 gigapixel image I shot in 2009, showing the entrance to the La Seu Cathedral at Palma de Mallorca. You can zoom in, watch it in fullscreen mode and discover the most tiny details.
I have taken various other gigapixel images of scenery, paintings and other objects. The superb resolution is also very helfpul if you intend to print a picture for a billboard or a long corridor e.g. in an airport. Your requirements are my challenge. Feel free to get in touch.