Twitter is an online news and social platform where users can post and react to short messages called 'tweets'. Almost everyone knows Twitter, so we will just leave it at that ;). Twitter used this onboarding email sequence to engage users to start using their platform.
After you subscribe they ask you to confirm your email adress. In email 2 they try to make you follow your friends so you will have some interesting stuff in your feed. In email 2 they show you how you can engage in a conversation on twitter, either by liking, retweeting or typing a reply. In email 4 they remind you to follow (more of) your friends. In email 5 they send you what some of your followers have been up to and in email 6 they start recommending interesting people for you to follow, based on who your circle follows. From email 7 and onwards they start sending daily emails that the follow the same format as email 5-7.
You can read more about Twitter here: twitter.com