Twitter is a great tool for teaching and learning. Many teachers have mastered it as a tool for expanding their personal learning network (PLN). If you are thinking, "Why should I care?" then you might want to take a few minutes to look over this article. It will help you get informed about what it is and how you might want to get started with it.
Fabulous overview - it's a chart, so it is a really quick way to get an idea about Twitter.
Thorough discussion of Teachers Using Twitter - this is a great resource and even includes 100 ways to use Twitter.