freak, Twitter's just the latest web phenomenon. Basically, you can type up any little thoughts or status updates that are under 140 characters. While I admittedly do have an account myself, I promise you're not missing that much.
Some celebrities have them and use them as a way promoting their latest projects, others are a little more chatty and use it to announce anything from what they're having for lunch to whatever random statements they feel like making. Wil Wheaton's another Trek alum (albeit not a Voyager guest) that has one too. As far as I'm aware, the TotallyKate Twitter is the closest any of the Voyager regulars have gotten to the site.