Twitter is a "micro-blogging" network where any post you make must be 140 characters or fewer. A post is known as a tweet. A tweet can include a website link (which may be shortened for you) a photo or a short video. If a twitter user follows you your tweets will be shown on their "timeline" in reverse chronological order (newest first) together with all other tweets from other accounts they follow.
Your twitter username (or handle) can be up to 15 characters long, has no spaces and is prefixed by @. You also have a real name which can be up to 20 characters long and can include spaces.
You can add some categorization to your tweet by including one or more words (or phrases with no spaces) prefixed by a #. These are known as hashtags which can be used as twitter search terms or to link you tweet with other tweets that are about the same subject. At any particular time very popular words, phrases and hashtags are highlighted on twitter as "trending", twitter users will often look at tweets about a particular trend and see you tweet even if they are not following you.
If your tweets are well received other accounts may retweet you (your original tweet will be shown to their followers) or favourited (liked) or replied to.
Twitter has a generally younger user base than Facebook with about 60% under 35 years old. Around a quarter of UK internet users have a twitter account. Users are fairly evenly split between male and female.
As you follow more and more people in an effort to gain followers your timeline can become full of tweets that are not relevant, using twitter lists can help you segment your timeline