There are a number of different ways to detect the existence of a wireless network, but they all boil down to being able to receive traffic from the target network. Some do this passively without sending any traffic themselves; others actively probe networks to get more information from them. Both types monitor network traffic to data mine information from the network.
To date, a number of methods have been devised to make network reconnaissance difficult on wireless networks. Some networks attempt to hide the network SSID from unauthorized listeners. Others try to do such things as limit access to the network to a specific list of MAC addresses, meaning you need to gather client MAC address information during the operations. Still others rely on proprietary protocol mechanisms to obscure some network data. The key difference between the various wireless reconnaissance tools is how they deal with the different roadblocks put up to make data mining difficult. It used to be that these sorts of countermeasures were available only on high-end enterprise class wireless products, so the casual wardriver could get by with simpler tools. Today, however, even the cheapest access points have these features, so having the right tools for the job is more important.