A sponsored server is a Minecraft server which is chosen by Minetrack to be officially endorsed and advertised on Minetrack.net.
They are chosen from the bidding list, the top 5 bidders each get there server of choice posted on at the top of Minetrack.net for the duration of one month.
After that month ends the next top 5 bidders are chosen from the bidding list and the cycle continues. Everybody can bid for their server to be in the top five spots however getting chosen is very dependent on bid amount.
Last month top 5 server bids: