As was rumored a couple of weeks ago, Microsoft's Windows 10 operating system will introduce a new browser codenamed "Spartan". This new browser will not replace the classic IE but is said to replace Modern IE. Since modern apps can be run on the desktop in a windows just like any other app, this means Spartan will run alongside classic IE if you wish.
As Neowin points out, the latest rumors suggest Spartan will be the new browser for Windows 10 mobile. One of the new "killer features" for Spartan will reportedly be an inking capability that allows you to annotate pages as you visit them, as well as share them with someone you know. This feature will work via Microsoft's OneDrive, which will store a copy of the annotated web pages so you can access it on any browser across major platforms.
Spartan also promises new ways to group tabs and may have custom skin support.
Microsoft’s new browser is said to be developed as a Windows Store app, capable of automatic updates, and will feature the same user interface across all device form factors. Internet Explorer will also be present as a legacy option, but Spartan will be the main browser moving forward.
Based on some blurry photos, Neowin created a replica of what the current version of Spartan looks like. Some more details about the UI can be read over here.