An iPad stylus pen will enable you to use your iPad's touch screen without placing your fingertips on it. This has the benefit of keeping your iPad screen smudge-free, and is also useful for people who have long fingernails, like to wear gloves, have large fingers, or anything else that might impair their ability to control the iPad with their fingertips.
The iPad does not come equipped with an iPad stylus which has caused some uproar. There are a number available to buy online, but since it is early days for these products, finding a pen that is effective can be a bit hit and miss. Check customer feedback to get an idea of which pen is the best, and which ones are not worth their price tag. The Boxwave capacitive stylus is highly recommended by lots of iPad users. It is effective with the iPad, gives great control, and looks smart too. Cheaper ones tend not to work as well, but that doesn't meant the most expensive ones are best. Expect to pay around $20 for a good quality stylus. It may be worth buying a cheaper back-up stylus to cover you if you loose your main stylus and need to wait for a new one to be delivered.