It seems to be a lttle bit like facebook, but not. You can post where you are and what you are doing. You can follow what other people are saying and doing. I haven't looked at it in a long time, but unless it's changed, a twitter is usually a short text. I never really 'got' it, so if I am way off base here, feel free to correct me!
I do understand a blog, a facebook page, an email or a letter, (remember those?), where you tell a little story about where you went and what you did.