I disagree when people say Twitter is a "micro-blogging" site. Twitter allows you to post small messages to their web site, but the way you use it is very different to how you blog. While Twitter does allow people to interact, it is much too difficult to use conversations in the same way that you use comments on a blog as a review and modification mechanism (as I explained yesterday).
I use Twitter in four ways.
In the olden days people had pagers and every now and then it would beep and a short message would show on its tiny screen. In more recent times SMS messages have filled this role. In both cases you pay for the benefit of these beeps and messages. This is a one way mechanism, the service provider sends the message and you read it. Twitter can be used in a similar way, but it does so without charge.
With Twitter you "follow" those accounts that you want to receive tweets from. When they tweet, the message will appear in your timeline: one message in a stream of tweets. A news item posted on twitter spreads very fast the source may have hundreds or thousands of followers, and many of them may re-tweet (ie copy the tweet to their followers) spreading the message to people who do not follow the source. It is no surprise that the newsmedia provide appropriate accounts (for example, @bbcnews, @skynews, @itn, @channel4news) for breaking news, and these usually link to an article to draw you into their website. Using Twitter like this effectively means that your timeline is a stream of news items, similar to the "tickertape" at the bottom of the screen on a TV news channel.
Some news tweets will have links to a complete article and I find that often it is easier to wait for an organisation to tweet a link rather than search for it myself (often websites have poor site maps and poor search engines). Some sites may tweet several times, others will tweet only when the site is updated. For example, I follow @DHgovuk because this account tweets whenever there are updates on the NHS statistics like waiting times. As soon as the statistics are released I get the tweet, read the page and then I can respond (re-tweet, or write a blog).
Some users will live tweet events. This is not quite like being there because the user will only tweet statements that they think are relevant. If you follow a user whose opinions you trust, this is a far better than attending the event because you do not have to experience the boring bits in between the relevant information. The true masters of this genre will also tweet explanatory, complementary or contradictory links to put the meeting into context. Live tweeting is similar to a sports commentator describing the action on the field.
I write blogs, when I write a blog I tweet a link to post. People who are interested in my blogs can follow me and when I tweet a blog post they will see this in their timeline and then they may choose to read my blog. If they like it, they can re-tweet my tweet (that is, copy my tweet to their followers) so that others can read it too. This is similar pamphleteering: you do not assume that people will buy a publication, and the only commitment you need is that people are willing to read your post.
Of course, this way of using Twitter is only useful if you have enough people following you, and the best way to achieve this is by tweeting interesting comments which will get re-tweeted. The followers of the people who re-tweeted you may decide to follow you and your followers list will increase. This means that communities are created, where people with similar interests follow each other. (You can monitor the people who follow you, through your followers list, and you can decide to "block" a follower, which means that they will be blocked from following you and will no longer see your tweets.)
This is where Twitter gets very powerful, but to do so it means that you, and others, should not "protect" your tweets. When a user protects their tweets they can have conversations, but only with the people they know and they miss the opportunity to talk with people they don't know. Conversations occur when you mention another user (prefixed with the @ symbol). If you start the tweet with the user name then that user and the users that follow both you and that user will see that tweet in their timeline. The person mentioned in the tweet will see it in their mentions timeline. You can also mention a user (again, prefixed with an @) elsewhere in a tweet, in which case all your followers and the mentioned person will see the tweet (the former in their timeline, the latter in their mentioned timeline). This is why you sometimes see a tweet prefixed with a dot (for example ".@anotherperson you and all my followers will see this"), it merely means that the message is expected to be seen by all followers.
Twitter is one way, but if people who you mention choose to they can reply back to you. This way conversations can start. Such conversations are not limited to two people. If one of the people in the conversation follows you, you do not have to mention them (unless you have replied with a user at the beginning of the tweet who they do not follow), but it is useful to add people into a tweet since the message will appear in their mentions timeline which will be less busy than their timeline. You can "send" a tweet to any of the people you follow (by mentioning them in a tweet) but whether they will read the tweet, or even reply to it, is another issue. However, I have found that if you tweet interesting and intelligent responses people will engage with you and consequently I have been able to have conversations through Twitter with people who normally I would never dream of engaging with: politicians, journalists, campaigners, academics and policy makers.
But note that you do not have to contribute to be part of a conversation. If you follow the people who are in a Twitter conversation then you will see the tweets in your timeline and this is essentially eavesdropping their conversation.
Twitter allows you to search for tweets with a specific phrase or with a contracted phrase called a hashtag. A hashtag is just a word or phrase (without spaces) prefixed with # (the name, I guess must be British who call the symbol # "hash" since Americans call it "pound"). Some Twitter clients give special treatment to hashtags so that, for example, if you click on the hashtag you'll see all current tweets which contains the hashtag.
Twitter search is not specific to hashtags, but it behaves differently depending on whether the search term is a hashtag or not. For example, if you are interested in NHS topics you can search for tweets with the #NHS hashtag. The result will only be tweets where the user has actually put the string #NHS in their tweet (they have consciously provided a hashtag rather than the word). You can also search for words other than hashtags, so if you search for NHS you will get tweets which contain NHS and tweets that contain #NHS and NHS, thus, you will get more results than simply searching for the hashtag.
If you are interested in a topic then you will have built up a community of people who you follow and people who follow you, all of whom are interested in the topic. You can widening this community through searches for a key phrase or hashtag. The result of the search will be tweets from people outside of your community and if you find that the tweets from one person are useful, you can follow them (to widen your community) and talk to them (through replies or mentions) and interact. Potentially, they will follow you, widening the community further.