There are two things at stake here:
- Crop factor: read up on
Crop factor - Wikipedia, the free encyclopedia and linked articles, they explain it better than I do
- Lenses specifically made for digital cameras. A digital filter reflects more light back into the camera body than a negative (you know, the old-fashioned chemical photography) does. This means that digital cameras are more sensitive to flares, which is compensated in "lenses for digital cameras". There is also another difference, which is linked to the crop factor. I'm sure that once you read the Wikipedia article you'll be able to understand it or ask more specific questions ;-)
Cheers,
Sybren