Tweets: You can send a total of 2,400 tweets per day. The daily update limit is further broken down into smaller limits for semi-hourly intervals. (And yes, replies and retweets are both counted as tweets)
Changes to account email: 4 per hour
Following: You can follow up to 5,000 accounts. Post this, additional follow attempts are limited by account-specific ratios. You only have a leeway of 10% of the number of your followers. So, if you have 10,000 followers, you can follow 5,000 + (10% of 10,000) accounts. 6,000 becomes your new Following limit. You are also limited to only following 1,000 accounts per day.
Oops, I hit a limit!
If you’ve arrived at a Twitter limit, you’ll see an error message telling you exactly which limit you’ve hit. In case it is a time-based limit, you’ll be able to resume your Twitter activity upon the time limit elapsing. Avoid coming too close to your limits though, because living too dangerously might just get your account suspended by Twitter for spammy activity.