Wireless speakers operate in essentially the same way regular speakers do but with the small addition of a transmitter, as opposed to wires, that carries a signal from the source to the speakers. Wireless speakers tend to have a range of no less than 150 feet of the transmitter, which is often much farther than you'll ever need. The signal broadcasts the same way an FM radio does and can travel through walls and furniture. Due to the "wireless" nature of the speakers, they're often powered by batteries or an optional AC adapter. Battery power enables you to move them around wherever you want--even into alternate rooms or outdoors--and still be able to hear the source signal.
When you start to play music or a DVD on your computer, the computer sends a signal to a transmitter. The transmitter then converts the signal into one similar to that of an FM radio--it's broadcast on a specific frequency to avoid interference from other objects. The signal is broadcast in a circular radius within the specified range, so any device that can pick up that kind of signal will pick up the sound from the computer. The speakers themselves have little receivers built into them that decode the signal into standard audio information for playing. The entire process happens in real time, so there's no delay between the time you hit "Play" on your computer to the time you hear sound coming from your speakers.
Wireless speakers also have the ability to decode discrete audio information like a standard home theater. This means that if your computer knows that a certain bit of sound should come out of the front right speaker, then it can broadcast it directly to that speaker. This is typically used when playing DVD movies. As long as the sound card on your computer supports discrete audio, you can get it from your wireless speakers.