Bluetooth is a high-speed, low-power microwave wireless link technology, it is the name for a short-range radio frequency (RF) technology that operates at 2.4 GHz and is capable of transmitting voice and data.
Bluetooth specification is quite large and daunting to the novice developer. But upon closer inspection, it turns out that although the specification is immense, the typical application programmer is only concerned with a small fraction of it. A significant part of the specification is dedicated to lower level tasks such as specifying the different radio frequencies to transmit on, the timing and signaling protocols, and the intricacies needed to establish communication. While there are a lot of API available which will do the talking to the lower system level communication and solutions, developer is responsibility is to use these resources and achieve the solutions the end user needs.
Here is the list of usages and development can be done in many ways:
-Keep contacts, calendars, and information in synchronize form on your computer, phone, and PDA ,Send files from a Mobile to a printer wirelessly.
-Keep your PC wire-free by connecting your computer to both a mouse and keyboard, and other devices.
-Print pictures by sending pictures from a camera phone, or digicam to a printer
-Listen to music on wireless stereo headphones streaming music from home stereo or similar audio devices
-Talk hands-free on a mobile phone as well as Laptop while driving a car. Pair a Bluetooth mobile phone to the audio system in a Bluetooth enabled motorcycle or to a Bluetooth enabled motorcycle helmet.
-In your city, listen to music wirelessly on headphones connected to MP3 mobile phones or MP3 players. Pause the music to take incoming calls and when the call is over, go back to your song.

Programming / Solutions for Bluetooth:
Based on the level and customer usage level, Bluetooth solutions can be classified into following types:
1.Bluetooth user interface development
2.Bluetooth communication interface development
3.Bluetooth browser web application development
4.Bluetooth utility development

Bluetooth programming can be done using following languages

1.Java/J2M3
2..NET

Also there are a lot of open source projects like Android where developers can create and publish applications.