WinterBoard

WinterBoard is a file replacement extension for iOS using Cydia Substrate, both of which are developed by saurik. It is often used for theming GUI elements, such as app icons. See saurik's website for more information.

It works by injecting replacement files dynamically but without actually replacing the files, though it appears to the iPhone that the files have been replaced. Should SpringBoard crash, the files will no longer appear to be the changed ones, except for images that iOS caches, such as the Notification Center and app switcher backgrounds.

WinterBoard themes are installed to /Library/Themes.

There was some confusion about whether saurik would update WinterBoard for iOS 7; for details, see his comment on Reddit from October 2013:

"There was never any question that WinterBoard would at least do things like theme icons, if that's what you are concerned with; the issue is that there are many more advanced features, and people were talking about how they don't work on iOS 6 currently, and the question came up about iOS 7: please understand that in the context of the actual discussion (context that blogs like iDownloadBlog ripped away) the idea is that WinterBoard hasn't worked correctly in years, and thereby certainly it is unlikely to suddenly start working on iOS 7. However, if WinterBoard works for you right now, to the extent to which WinterBoard is even possible on iOS 7, you will be at least sort of happy with its functionality at release."

WinterBoard was indeed updated to support iOS 7.

WinterBoard's predecessor was SummerBoard; this August 2008 article about the release of WinterBoard has context about SummerBoard.