If you notice problems with your favorite Twitter app, it could be because Twitter has shut down API v1. An API is a programming interface that allows others to use their data services when building apps. In the early days of Twitter, they wanted all the attention they could get and this API made it easy for apps to pull in Twitter content.
Today’s retirement is unlikely to affect any popular apps — since developers have been well aware of the impending transition for some time, and can largely continue affected — but it is a clear milestone of Twitter’s once open ecosystem becoming more restricted.
One popular app that will no longer function is TweetDeck, but others have either become more expensive or shut down completely because the restrictions on the new API meant is was impossible to function profitably.
You’ll find a more complete description of this issue at The Next Web.