If you use the official Twitter app for Android... dear God, why are you using the official Twitter app for Android? Stop reading this story and go download a better client. If you simply can't be torn away from mediocrity, you'll find a few new options the next time you update Twitter in the Play Store. Chief among them is a new crop tool, with easy shortcuts to Square ("Instagram mode"), Wide, and Original.
According to Twitter's blog post on the update, you should see reminders to @reply friends before you post, which didn't happen for me (maybe you need photos of people). Twitter also says that if you do the standard "pull to refresh" move and there's nothing new in your main Twitter feed, you'll be given recommendations on who to follow, trending topics, or recommended tweets. Again, I didn't see this when I tried out the new version of the app.
Other changes in this update appear to be minimal. You can download Twitter in the widget below. Or, you know, go find something better. Either one works.