Bluetooth is a short range wireless technology basically to replace cables. Eventually many devices will be BT...some examples are/will be your cell phone, hands free headset, PDA, laptop, gps etc. Short range is around 30 feet or so. It's being term PAN for personal area network.
WiFi is longer range wireless LAN (300 feet) which has wider band width for data.
Both are relatively inexpensive ($100 for WiFi router/access point to transmit + $100 for card in handheld device). BT is bit cheaper, devices will come with it built in or can add on card for $50-100.