Twitter - enlighten me, please.

I'm off sick today. I don't feel too bad if I just sit still, but if I get up and try to do anything I feel like death. Having said that, I dragged myself downstairs to watch Project Runway, and post this.

It seems daft to ask a question in a journal that almost no-one reads, but on the off chance a knowledgeable person is passing by, could somebody please explain Twitter to me? I know that you post messages by text - yes? But I just went and had a look at Stephen Fry's twitter and I couldn't see any of the comments, which is half the fun of lj. Do you need to have a twitter account yourself to see them? I often see posts about Tom Felton's twitter, and what really made me think to try and suss it out is that I saw something about the Phelps twins on twitter - I want to see if they ever say 'I'm in the post office...'