From Wikipedia:

Twitter is a free social networking and micro-blogging service that enables its users to send and read messages known as tweets. Tweets are text-based posts of up to 140 characters displayed on the author’s profile page and delivered to the author’s subscribers who are known as followers. Senders can restrict delivery to those in their circle of friends or, by default, allow open access. Users can send and receive tweets via the Twitter website, Short Message Service (SMS) or external applications.

The reason that Twitter sticks to the 140 character limit is because it began as an intermediary between SMS text messaging and the web. Text messages can be 160 characters long. The 140 character limit allows 20 characters for the addition of the screen name of the person tweeting.

To enable tweeting from your mobile phone, follow these steps:

1. Go to Twitter.com and either sign in or make an account.
2. At the top of your Twitter home screen, click Settings.
3. Click the Mobile tab. (If you’re logged in, click the previous link to go directly)
4. Follow the on-screen instructions. In the future, you can send tweets via text to 40404 at standard text messaging rates.
   

Once logged into Twitter, all you have to do to tweet is type something into the box at the top of your home page (under “What are you doing?”) and click “Update.”

Some lingo:

Tweet: Term for a 140 character message on Twitter.

Follow: This is the term for someone you’ve chosen to watch on Twitter. When you’re logged into the site, you can follow another user by clicking the “Follow” link under their image and username. ie: To follow Ashton Kutcher, go to http://www.twitter.com/aplusk and click “Follow.”

When you follow someone their tweets appear in your main Twitter feed. In other words, when you’re logged in and visiting Twitter.com you’ll see all the tweets from people you follow in the order of when they were posted.

@username: If text is preceded by the @ symbol in a tweet, it creates a link to an account with the name following the @ sign. For example, if my friend is on Twitter and his page is at twitter.com/afriendofmine, I can tweet @afriendofmine and it will automatically create a link to his page. It would look something like this when posted:

“Hey @afriendofmine, this is a link to your Twitter page!”

DM (Direct Message): This is a way to send a 140 character message to a friend. When viewing someone else’s Twitter page, click “message” under Actions to send a DM. You can also send a DM from your own inbox by clicking “Direct Messages” on your profile page and using the box at the top. Either works. DM’s do not appear in any twitter feeds. They’re completely private. DM’s can only be sent to people who are following you and only those you are following can DM you.

Favorites: When you mouse over any tweet a secondary menu pops up. Off to the right is a faint star. Click on this star to mark a tweet as a favorite. Then when you come back to your account you can click “Favorites” on the right to view all of the tweets you’ve marked in the past.

RT (Retweet): Often times you may see a tweet that you like and want to share that with people that follow you. The way to do this is by retweeting. Retweets often follow the following format:

RT @initial_tweeter: This is the tweet I’m retweeting http://bit.ly/3VJVjN

Now all your followers have the link to the page you liked, a link to the person you’re following that tweeted it, and the original tweet.

Trending Topics: One of Twitter’s most powerful features is the Trending Topics section. This is a live feed of what people are posting about most often, and it isn’t uncommon for major news to make it here before you’d see it on CNN or Fox.

#hashtags: Hashtags are a way for you to mark your tweet as part of a larger group. They frequent the Trending Topics section. For example, after the recent Fort Hood shootings, the following tweets would both show up if you searched Twitter for #forthood:

My thoughts and prayers are with the victims. #forthood

#forthood They just caught the perp! Hoping for swift justice.

If a lot of people used the #forthood hashtag, it might eventually become a trending topic.

TweetUp: Taken from “meetup,” this is what you call it when Twitter users meet eachother offline.