@ddljohn-- They can be the same thing if the wardriving is done to access other people's wireless networks. So we could say that wardriving is a type of sniffing.
Sniffing is basically locating WiFi networks. It could be done legally to access a public network, like at a coffee shop. But it doesn't involve driving around. Many of us do sniffing with our smart phones when we need to find a cafe or restaurant to check our email. But there are people who do illegal sniffing, where they not only locate wireless networks but also use them and steal data that's transferred via that network. It's almost like phone tapping.
Wardriving is less innocent, because it almost always involves illegal activity. And it's a more active form of sniffing because the individual is literally mapping WiFi networks in a given area for some purpose.