There have been many rumors of Microsoft developing a new web browser called Spartan. Today the rumors were proved true with Microsoft announcing the new browser at their Windows 10 Press Event. The new browser is being called Spartan and will be alongside Internet Explorer in Windows 10. Spartan is just the codename for now and will likely be changed by the time of release.
Spartan was developed using Microsoft's Trident rendering engine which is also used in Internet Explorer. It's not simply an update to Internet Explorer. Instead it's an entirely new web browser.
The new browser includes some innovative features. For example it has some collaboration features. You can collaborate with others by by drawing on a page directly. You'll be able to click anywhere on the page to comment. The browser also has a reading mode that downloads articles for offline reading, giving you the ability to read websites when you don't have access to the internet.
In addition to bringing Cortana to the desktop, Cortana will also be included in Spartan. It'll learn about you over time and then give details about things it thinks you are interested in.