Simlpy Iphone is a phone whereas Ipad is like a Gadget with several features. Got few more here- http://www.differencebetween.net/object/gadgets-object/difference-between-ipad-and-iphone/
iPhone is mobile phone but iPad is not your phone, you can't call any one.iPhone is small in size whereas iPad is large in size iPad favors window bit iPhone does not favor any window.
The main diff between iphone & ipod is... iphone is a cell phone but ipod is a device used to listen songs....
The biggest difference is that iPad 3g has diffirent Baseband Chip then iPhone. iPad Wifi dosn't have it at all. Read iPad News to always stay tuned on this products.