That's a comment I hear whenever I discuss the idea of Twitter - a microblog.
A microblog is simply a smaller version of a blog - its posts are more succinct, and its content smaller.
Posts (or tweets) are short (140 characters maximum), and most commonly done from your mobile phone.
Other uses might be:
research, storage and/or networking...
use it to save / share weblinks about your subject
use it for short notes to self
utilise twitter to post photos
use it to network and connect with others
Your Twitter account can be set to public or private
Consider what's best for you.
*useful Twitter safety and security information*