The i-pad is a computer tablet - like a laptop, but just the tablet. It is about the size of a book and doesn't open (think a giant cell phone). It is designed to browse the internet, do calculations and run software like a computer, but it is smaller, lighter and has a touch screen.
The idea behind it is that it can be easily carried to and fro and used on the go. So, students can easily bring it to class, scientists can bring it from office to lab to factory. It can be used while waiting in a waiting room, cafe or curled up in a chair.
Apple has brought the tablet to mainstream consumers. Jon ordered them this morning, the apple store opened at 8:30 this morning. Now, Jon has to wait until April 3rd for it to arrive!