The 140-character limit on tweets originated with the short message service (SMS), which has a limit of 160 characters. Twitter allocates the 20-character difference for usernames. Senders may restrict delivery of their tweets to their followers.

Other users can mention, reply to or "retweet" (RT) tweets as other functions of Twitter.

Even though each tweet is limited to 140 characters, there is no limit on the number of tweets each user can post. So, if users want to convey more through their tweets, they can post a series of tweets back to back.

In August 2009, a market research firm categorized 2,000 tweets into six categories over a two-week period, as follows:

• Pointless babble: 40%
• Conversational: 38%
• Pass-along value: 9%
• Self-promotion: 6%
• Spam: 4%
• News: 4%

Social network research expert Danah Boyd replied to this survey by pointing out that the category labeled "pointless babble" is not just a group of pointless talkers; instead, it can be described as social grooming or peripheral awareness - that is, wanting to know what other tweeters are thinking, doing and feeling on a constant basis.