can someone explain twitter to me?

[Woody.]

are there any notifications that someones replied to you?

[Mark]

Twitter is a social networking site. Anyone can create an account and start typing updates about their lives. Once you have typed these updates people who follow you then receive the message.
In a nutshell its like a SMS message sent to a group of people in your inbox.

[Sheldon]

Twitter is currently the hottest platform on the rapidly expanding social networking landscape. Twitter is a website, of course, twitter.com, and it provides users the opportunity to send free, short posts featuring content spanning the information spectrum, and to read the posts of others.