It's 4x the size, you can connect to the internet from any where in the world (at a price of course) and there's a larger library of applications. If you have an iPod touch don't buy an iPad, but if you get rid of your iPod touch you should get an iPad.
A iPad is a larger than the iPod touch
A iPad has eBooks but if you have ios 4 on your iPod touch then it is the same.
A iPad cost more than the iPod touch
A iPad is alittle faster than the iPod touch
With the iPad WiFi+3G you can go online where ever you want.
If you already have a iPod touch then you don't need a iPad