Previewed at WWDC, launching in the fall.
Rumored Split-Screen Multitasking Feature for iOS 8 Not Ready for WWDC Preview
Designed for the iPad Air and possibly a larger-screened iPad to be released in the future, split-screen multitasking would allow two apps to be displayed at the same time on a single screen while in landscape mode. It might also include a feature that allows users to share information like links, images, and text between two apps.
The iPad's inability to display two apps at once is a lack that Microsoft has used to heavily advertise its Surface tablet, which does have the ability to display multiple apps on the screen at one time.
While the multitasking feature will reportedly not make its debut at WWDC, it is still slated for inclusion in iOS 8. It appears multitasking will be limited to the iPad Air at launch, and it is unclear whether the smaller Retina iPad mini and iPhone might also gain the functionality.