Whats the difference between reply and "@" on twitter?

[HelloGoodbye]

Im a bit confused. Because when you follow a celeb and u do "@ w/e their name is" then they'll see ur tweet right?
But same goes when you reply to their tweet right O_o? Dont they see it aswell?
So why does twitter have both? Their must be a difference between those two...but cant figure it out.
I mean whats the difference. Im very confused thanks so much!

[Sam Chin]

The reply button is a quicker way for you to have the @ sign to your post, and in order to avoid mistakes while typing in the username. Sometimes some people tend to misspell when they do the manual way of replying. Username and full name can be totally different on Twitter. So it's better for you to click on "Reply" button directly to avoid misspelling.