Skitch is a Mac-only screenshot taking application. Think of it as an Mac alternative for the popular Windows screenshots (with annotations) application: FSCapture (my personal favourite).

Basically, any application designed for taking screenshots can do that, and the difference is made by the extra options such as easily highlighting some text, adding arrows and so on, and at this, Skitch is the best I've seen. You can even call it a lightweight image editor because it can be used for that too.