Here's another back-to-basics article that will straighten out this question once and for all.
If you type “What's the difference between an iPad and an…” into Google, it auto-suggests that you mean “What's the difference between an iPad and an iPhone“, which implies that there are more than a few people searching for the answer to just that question. And is it turns out, there are more similarities than differences which makes it a fairly valid question.
The first obvious difference is their screen size – the iPhone has a 3.5″ screen and the iPad has a 9.7″ inch screen. The iPhone has a 640×960 resolution at 326 ppi (pixels per inch) while the iPad has a 1024×768 resolution at 132 ppi.
They both run the same operating system (iOS).
Of course, the single largest difference is that the iPhone is a cell phone and can make calls whereas the iPad can't (yet?). The mere fact that the iPhone is a mobile device is by itself a large differentiator. However, the iPad 2 now has a front-facing camera to allow you to video call over Wi-Fi and there is an array of VoIP apps for iOS that can actually allow you to make calls off the iPad, should you wish. Of course, we wouldn't recommend you try holding it up to your ear (awkward and heavy)!
In terms of hardware, specifically sensors and inputs, the latest generations of iPhone (4) and iPad (2) are actually quite similar. Both have multi-touch touch screens, proximity and ambient light sensors, 3-axis accelerometer, 3-axis accelerometer, digital compass and GPS.
In terms of battery life, both vary depending on use (Wi-Fi, calls, watching movies, etc.).
Due to the significant differences in screen sizes, many app makers produce different versions of their apps for both devices, and in some cases, some apps are made exclusively for one or the other. But what you'll find in most cases is that apps designed for the iPhone will work on the iPad (unless they're specific to mobile phones), and the iPad has a feature that will double the size of the iPhone app displayed on the iPad so that it fills the screen.
Pretty much, what this really boils down to is whether or not you need a phone or not. If you don't, and want to get the added benefits of a larger screen with more real estate for different and interesting apps, the iPad is for you.