How do sites like yahoo get their NFL information. I wanted to tap into NFL information for a web app.
I believe Stats, Inc. is the official provider of NFL stat data
Most sites like Yahoo or Sports Illustrated use an outside service like Stats Inc.. It is quite costly (1000's of $) to get info from them however so unless you have a viable business model already in place it can take quite a bit of capital upfront.
A simpler solution is one like using XML Team. They have an easier "pay only for what you use" model and is quite cheap.
Not so simple but effective (and free) is good old screen scraping.
This is what things like RSS and JSON are made for. The NFL updates are driven via these technologies and they're often intended to be repurposed. It's not going to give you access to everything Stats Inc. will give you access to but for typical game updates and tickers and what not, this should be fine.