“1. Tango is not a major update.
2. Tango is one of the update belonging to Mango.
3. Apollo would be the next major update.
4. Tango is mainly for developing countries including China and India. These countries were neglected before and Microsoft would like to provide more Bing Services to them through Tango.
5. OEMs would be selling cheaper Tango devices in these developing countries.”
Tango devices will possibly hit markets with a less capable processor and screens with lower reoslution. This should allow Microsoft to reach out to a wider audience with cheaper price tag and presence in both the low/mid and high-end segments. But Redmond is not going to be the first to set its foot there - Android is already pushing the boundaries of low-end smartphones and even Apple is rumored to roll out a cheaper iPhone this fall.
source: We Love Windows Phone via WMPowerUser