Whilst I have no 'hands-on' experience of jammers, from what I've read, this is a big subject...
Could anybody please elaborate how a jammer actually works?
If you are talking of radio reception jamming, this means making the reception of radio signals unintelligible or impossible in a particular region. At least two and many other ways to do this,
1. Is to use the original transmission and retransmit with a time delay or several time delays with antennas beamed to the place you want to cover, making the reception tedious in the extreme.
2. Is to use the transmission frequency and beam 'white noise' at much higher power than the original into the chosen region thus drowning out the original signal.
Does it have a particular bandwidth of operaton or it jams all the frequencies in a particular region?
It really depends on who/why the jamming is being done. There may be very good reasons why only one or several stations are being jammed and that others may be allowed to be received. So, it can be selective - If you wish to deny a region communications entirely then you would use several high power, broad spectrum jammers and antennas. If you wish to obscure one station then a high power, restricted bandwidth transmitter would be appropriate.
Does that help?