A bored mom's best friend. Hahaha. Honestly, it does make for good networking if one has a blog they want to promote. I've made some pretty interesting friends there!
I usually explain it as one on-going chat room where people make short (140 character) statements to each other. You choose whose statements to read, and other people may or may not choose to read your statements (tweets) by "following" you. If you choose to tag your statements with a topic (hashtag), it creates a link and then you are able to see many more statements on the same topic. One popular topic is #bfcafe where breastfeeding women chat with each other about their experiences.
I use it mostly to chat with local people I know and I'm meeting a few local moms. I follow some newsblogs and feministblogs, and when they post links it means I have interesting things to read while nursing. Its also a chance to be funny. I like being funny, though I'm mostly miss on the hit or miss scale. :P