Krita is an image editing and painting application written for KDE under Linux. KDE applications can be squeezed into normal Linux and Krita is also available in an experimental Windows version. One thing jumps out in Krita to make it important. Gimp is the world's most popular free open image editor and can run on every useful operating system. Gimp is restricted to 8 bit editing. Krita can edit in 16 bit and is close to working on the same operating systems as Gimp. Cinepaint is in the same position as Krita. One of the two could be your replacement for Gimp when you buy a pro camera with RAW output and need 16 bit editing.
Read about Krita at www.krita.org.
Painting, not editing
Krita is focused on painting, not image editing. Krita is not a replacement for Gimp or Photoshop for professional editing of photographs. Krita is not a replacement for Inkscape when creating diagrams using SVG. If you want to paint then try Krita. If you want to paint most of the time and occasionally do other things, you may find Krita does all the other thinks you need. The following work was created by Enrico Guarnieri using Krita.
Krita does have filters and other types of add-ons. Gimp set out to be plug compatible with Photoshop for basic plug-ins, giving Gimp a quick start. Krita plug-ins appear to be different enough that you cannot use another brand and similar enough to quickly acquire the same basic range of plug-ins as the other applications.
KDE, not Gnome
Gnome is the most common user interface in Linux. KDE is a popular alternative. KDE can run under Gnome, although you will waste a lot of memory on the duplication of code. I do not use KDE because every time I try to use KDE, important applications crash. Other people say they find KDE better than Gnome.
Krita can run under KDE under Gnome on Linux. I tried it once a while ago when trying to find a 16 bit replacement for Gimp. All the KDE applications installed hundreds of megabytes of KDE and all of them crashed. I gave up. All of those KDE applications now have new releases and some of those new releases might work under Gnome.
Krita and CinePaint are both experimenting with versions for Windows. I am not about to waste time testing their experiments with Windows versions. I might test one of the Windows versions when they reach the beta stage. My first priority is image editing and CinePaint looks closer to what I want and CinePaint looks closer to a working Windows version. (Cinepaint was originally based on Gimp).
Krita or CinePaint for painting?
Painting images is something I do more often than Hollywood releasing movie versions of The Thing but less often than Hollywood releases versions of Planet of the Apes. When I do paint something on to an image, I like the Inkscape SVG approach of creating a movable resizable element. If painting is your first priority, find someone who paints what you want to paint and ask them what they use.