There's been talk for a while that Microsoft was going to make some big changes to Internet Explorer in the Windows 10 time frame, making IE "Spartan" look and feel more like Chrome and Firefox.
It turns out that what's actually happening is Microsoft is building a new browser, codenamed Spartan.
Windows 10 (at least the desktop version) will ship with both Spartan and IE 11, IE 11 will be there for backward-compatibility's sake. Spartan will be available for both desktop and mobile (phone/tablet) versions of Windows 10.
Spartan is just a codename at this point. don't know what Microsoft plans to call this new browser when it debuts.
Microsoft may show off Spartan on January 21 when the company reveals its next set of Windows 10 features. Not sure if Spartan will be functional enough for inclusion in the Windows 10 January Technical Preview and mobile preview builds that are expected to be available to testers in early 2015. It may not show up in the test builds until some point later, they say.
Will Microsoft end up porting the Spartan browser to Android, iOS and/or any other non-Windows operating systems? Not sure. The IE team said a few months back that Microsoft had no plans to port IE to any non-Windows operating systems. But Spartan isn't IE. And these days, Microsoft is porting much of its software and services to non-Windows variants. So there's a chance that this could happen somewhere down the line.