Why is 'Sprint' written in ebay descriptions of iphones?

What's Sprint? I'm looking to buy a new iphone and the majority of the ones on ebay have "sprint" written in their title description. What does this mean? Is it a mobile carrier or something? Like a version of Telstra?
Thanks in advance!

Added (1). Oh, and what's "Verizon" as well?
Are these iPhone's still reliable?

These iPhones are USA versions locked to that carrier, The largest mobile carriers in the USA are; Verizon, Sprint, AT&T, and T-mobile. Also, Verizon and Sprint iPhones might not work outside of the USA.

Sprint is a major carrier in the US.

Sprint is a CDMA network in the US. Yes it is a carrier. Verizon is the same thing, different company of course.

They of course have the same reliability as any other iPhone.