Followers are people on Twitter who follow you and therefore see your Tweets. If someone follows you:
- They'll show up in your followers list.
- They'll see your Tweets in their home timeline whenever they log in to Twitter.
- You can send them direct messages.
If you follow someone on Twitter:
- You are subscribing to their Tweets as a follower.
- Their updates will appear in your Home tab.
- That person is able to send you direct messages.