Question: When someone puts RT before your name on a tweet, what does that mean?
Answer: RT means "retweet". It's similar to forwarding an email to someone else, but in this case you're forwarding a tweet to anyone who is following your updates on Twitter.
RT's are used when you find something interesting that someone else said, and you want your Twitter followers to read it (because your network is probably different then the network of the original Twitter poster).
Here are a few other useful Twitter shortcuts:
Adding d in front of a twitter address (e.g. d@rickysays) will send your message directly to that person in the form of a private direct message that won't be visible to anyone else.
Direct messages are great for taking Twitter conversations offline that are private or too detailed for a broader audience.
Editing a Tweet
Sorry, there's no way to edit tweets. Once they're out there, they're out there.
But don't stress too much about things like typos or abbreviations. Twitter users are very good at figuring out what you're trying to say, even if you've highly abbreviated your message.
At the time of this writing, Twitter doesn't offer any built-in support for posting pictures. However, you can use a free service like TwitPic to store your photos and post links in Twitter.
Once you create your TwitPic account, just enter your Twitter user name and password to get an email address you can send pictures to as file attachments. The email subject is the tweet that will show up in Twitter (no email body needed), along with a link to the photo that you attached to the email message.
If you're using Twitterrific on your iPhone, it also has a built-in feature for posting pictures as a tweet - using existing pictures or letting you take a new one on the spot.