I will be buying an iPad in 3 days, and I have to decide whether I want to get the WiFi one or the one with WiFi and 3G/4G. I do not understand what 3G and 4G are. Could you explain that to me? And I also wanted to know if that has anything to do with a certain card that is placed in the iPad. Well, I heard there was some sort of mini-card. What is that anyway?