WiFi module is the instrument that can be used to send and receive WiFi signals free of charge. There are WiFi modules in mobile phones, MP4 and laptop computers.
WiFi is a way of accessing network, and it can also be understood as a technology. Wi-Fi is a technology that can connect personal computers, handheld devices (such as PDA, mobile phone) and other terminals in wireless way. Wi-Fi is a brand of wireless network communication technology, held by Wi-Fi Alliance (Wi-Fi Alliance).
In a word, if your digital devices (such as notebook computers, mobile phones, MP4 and so on) have WiFi functions, and there are WiFi wireless signals around it, you can access the WiFi network and don't have to charge. Also, in KFC, McDonald's, Starbucks and other places, there are basically WiFi signals, where you can get free internet access. If there is a password in the restaurant's wireless network, you can ask the waiter for the password.