Ubuntu Netbook Edition is a version of Ubuntu with the UI optimized for small screens. Canonical has been maintaining the Ubuntu Netbook Edition since a smaller screen requires special optimization to make optimum utilization of the available screen size.
However, starting from the next release - Ubuntu 11.04 "Natty Narhwal" - the desktop version of Ubuntu is adopting the Unity Interface as well. The Unity Interface was actually developed as an interface for netbooks and it is used in Ubuntu 10.10 Netbook Edition. Since the desktop version is also going to use this interface, there is no point in maintaining a separate edition. So, Canonical has announced that starting from Ubuntu 11.04, Ubuntu Netbook Edition will be merged into the Ubuntu Desktop Edition and called simply as Ubuntu.
In the last release cycle, Kubuntu also merged its Netbook Edition and Desktop Edition into a single image. However, unlike with Ubuntu, Kubuntu still has different UI for netbooks and desktops. Which UI to use by default is decided according to the screen size during installation.
Ubuntu Server Edition will however continue to be maintained - but Canonical has decided to remove the word "Edition" from the name and it will be known as Ubuntu Server. This means that the number of editions of Ubuntu has now been cut down to two from three.
According to Canonical, removing the "Editions" from the names will help the Ubuntu brand and create less confusion among users. For example having a Desktop Edition confuses people into thinking that there would a laptop edition as well.
So, when Natty is released next month, users no longer have to download separate images for their desktops/laptops and netbooks. A single image of Ubuntu 11.04 will work on both.