The iPhone includes its own notes app, but it does require that the onscreen keyboard is used for typing. When you're in a rush, pressing tiny virtual keys can be a problem and is likely to lead to mistakes. For many people the first thing that springs to mind when making a note of someone's phone number or jotting down another snippet of information is to reach for a pen and paper. The Notes app brings this concept to the iPhone screen.
In its current incarnation the app is very basic, enabling you to use your finger to draw and write messages and doodles. It is only possible to create single pages notes, but this limitation has been worked around by providing the ability to take a screenshot of notes which can then be accessed through the camera roll. This is ideal if you are taking longer notes or want to be able to email a note to someone.
Future versions of the app promise support for different pen sizes and colours, and multi-page notes. This is already a very useful app, but there are even better things to come.