What does Extended mean on my iphone?

Today I noticed at home instead of the usual sprint it says extended. What does this mean? Am I being charged extra? I'm in area of poor signal. I'm on a iPhone 4S ios 7 in Texas if that matters.

You may be on something too long or something you are doing is letting you do it for longer.

You have run over your limit!

You're simply on an "extended" network because of poor signal strength. You're not being charged extra. It's just an indication that there are no Sprint towers in your area, so you're being serviced by another tower owned by a third-party network with whom Sprint has a partnership or agreement for network sharing.