On Twitter, there are a few different types of tweets:
A message posted to Twitter containing text, photos, a GIF, and/or video. Basically, these are all tweets that do not start with an "@" symbol.
A Tweet containing another account’s Twitter username, preceded by the "@" symbol. For example: “Hello @Agorapulse!”
A reply is when you respond to another person's Tweet - you can respond to another Tweet from the INBOX or LISTENING section in Agorapulse.
Then there are a few other types of tweets, you can learn more about them here.
When you post a Tweet beginning with an @username, only people who follow both you and the account you are mentioning will see the Tweet in their Home timeline. If you’d like all of your followers to see the Tweet, the best way is to Retweet, Quote Tweet your Tweet or to put a dot "." symbol just before the username, just like this ".@username My tweet"