Twitter used to provide their anlytics only to advertisers and power users but as of right now, they're open to everyone!
So now you can find out just how many impressions your Tweet about your breakfast generated and what level of engagement you got for that seemingly casual message that actually took you about 20 minutes to compose perfectly.
If you're logged in you can just head straight to analytics.twitter.comwhere you'll be able to see all the stats on your Tweets and replies and keep track of any promotions you were running. You can also take an exhaustive look atyour followers including breaking down their interests and location.
How much use this will be for non business users remains to be seen but if nothing else its a tiny glimpse into the mechanisms behind social media, and probably another way to verify that posts about cats are still the ultimate way to win at the internet.