Bluetooth and WiFi are two completely different things. They are both ways to connect devices together over radio links, but they are two different standards for doing so, and they have different purposes.
WiFi (also called 802.11 wireless) is for networking multiple computers or computing devices. You use WiFi when you connect your laptop to the Internet at home or at a Starbucks.
Bluetooth is for connecting accessories to a computer or mobile phone: mice, headsets, keyboards, things like that.
They're both wireless, and they both use the same radio frequencies, but they're different tools for different jobs.