Photos, videos, GIFs, polls, and Quote Tweets no longer count towards the 140-character limit, freeing up character space. Previously, media content took up approximately 24 of the 140 available characters.
Twitter originally said that all @names, such as @MacRumors, would also no longer count towards the 140-character limit, but that change does not appear to have been implemented. There is no mention of it in Twitter's announcement and @names are still counting in the character limit.
Say more about what's happening! Rolling out now: photos, videos, GIFs, polls, and Quote Tweets no longer count toward your 140 characters. pic.twitter.com/I9pUC0NdZC — Twitter (@twitter) September 19, 2016
It is not clear if Twitter plans to eliminate @names from character count in the future, but the company has said it is exploring additional ways to allow people to better express themselves, so more changes could be in the works.