Several people have asked me about types of tweets recently, so here's the rundown:

  1. If the first letter of a tweet is d or D, it's a 'direct message' -- follow with space and name, like "d mscrochety Hey, babe!". This only goes to the recipient, and nobody else can see it. Screwing up DMs, and making unintentionally public comments, has a storied history already. If you try to DM someone who doesn't follow you, you'll get an error.

  2. '@ mentions': Any time a username appears in a (public) tweet after an @, that user sees the message, even if they don't normally follow the poster.

  3. '@ messages' or '@ replies': Anything with first character @: These are public (visible on your page & in searches), but not sent to your followers unless they also follow the recipient specified after the initial @. So if I tweet "@mscrochety Hi there", my followers who don't know @mscrochety won't see it, but people who follow both of us will. If you are addressing a person but still want others to see your tweet, put an extra character such as a space before the @ to convert it to a 'mention' seen by all your followers.

  4. Public tweets: If the first character is anything but @ or d, anybody who follows the poster sees them. They also show up on the poster's feed page (e.g., http://twitter.com/reppep) and in searches.

  5. Hashtags: To make it easier for people to find your posts, you can use tags which start with the # character to identify topics. These are popular for people in a venue to collect their tweets, or for a larger meta-conversation. Hashtags should not contain spaces or punctuation, and are normally lower-case.

When you want to echo what someone else said, in current clients (including the twitter.com web interface), you can use the retweet button. This shares the original tweet with your followers, even if they don't follow the original poster. There's an older convention of starting with 'RT '.

For Twitter applications, including photo and video integration, see twittereye.