How does 3g4g cell phone jammers work?

After heard the news that one man in Florida was fined of  $48,000 by operating jammer ,you may think whether cell phone jammer is legal in your country.

As far as we know,mobile phone jammer is not a normal product.

How does 3g4g cell phone jammers work?
By emitting radio frequency waves that prevent the targeted device from establishing or maintaining a connection, it can block all radio communications on any device that operates on radio frequencies within its range.

Here brings you some countries for example.
  Armenia : legal
 Australia: illegal to operate, supply or possess
 Belgium: illegal to sell, possess and operate (licensed part of the spectrum).
 Canada: illegal, except by federal law-enforcement agencies who have obtained approva
 People's Republic of China: Used by the Education government department as a method of thwarting cheating in schools. During major end of year exams, mobile phone signal jammers are used in areas surrounding high schools to prevent students inside from receiving calls or text messages, which may be used for illicit purposes. In some municipalities however, rather than the use of jammers, mobile signal towers close to schools are temporarily shut down for the duration of the week as exams are in progress.
 Czech Republic: illegal.
 Denmark: illegal.
 Egypt: illegal.
 Finland: illegal.
 France: France legalized spy cell-phone jammers in (movie) theaters and other places with performances in 2004. Abandoned due to complaints regarding emergency calls. Still legally used inside jails.
 Germany: illegal, but installation in jails has been proposed.
 India: legal, Government, Religious Places, Prisons and Educational Institutions use jammers.
 Iran: illegal to operate for civilians but allowed for police forces and military. It is however legal to own such units, which can be bought in electronic markets without a licence. In most jails, libraries and university classrooms such jammers are already in use. In the 2009–2010 Iranian election protests, police forces used cellphone and Bluetooth jammers.
 Ireland: illegal to operate. Legally used inside prisons by the Irish Prison Service.
 Italy: technically not illegal to own, but illegal to operate, since the Italian law specifically prohibits the disturbing of radio and telephone communications. GSM jammers are however legal to be used in places like hospitals, churches, movie theatres and other places with performances, and other buildings where and when the use of mobile telephones may result in a leak of sensitive information: on such occasions, jammers are legal as long as their operation doesn't interfere with electronic medical equipment (such as pace-makers) and allows mobile phones to make emergency calls. Tri-Band Jammers are reserved to, and in use with, the police forces and are being experimented in prisons.
 Japan: illegal to use, but legal to own. Buying of mobile short range versions is allowed. Use of fixed high output jammers with long range is illegal, with fines of up to max $250,000USD and/or 5 years in prison.
 Mexico: legal inside jails, often used also in churches and hospitals.
 New Zealand: Illegal to sell, manufacture or use.Legal inside jails by department of corrections.
 Norway: illegal to own and operate. The police and the military can use jammers in situations in which it is necessary.
 Pakistan: It's Illegal to Operate a Jammer without the N.O.C from Ministry of Interior and Ministry of Telecom, Further State bank of Pakistan banned the installation of same in Banks, PTA (Pakistan Telecom Authority's Enforcement Division contonously monitors and receives complaints and takes actions to remove Jammers which are installed without N,O.C.
 Poland: legal to trade and own, while not operational; illegal to use in public (licensed part of the spectrum); illegal to cause interference to 3rd party communication (without notice and permission); legal to use on own property with small power (both above clauses apply).
 Slovakia: illegal.
 Sweden: illegal. Legal inside jails and for military use.
 Switzerland: illegal.
 Turkey: illegal. Only the police and the military use jammers.
 Ukraine: legal, planned to be used in schools
 United Kingdom: illegal to use, but legal to own. Installation in jails has been proposed
 United States: Cell phone blocking devices are used by federal officials under certain circumstances. Privacy rights of property owners may affect the policy and application of law within buildings. The FCC may issue a permit that waives the law for private use. For radio communications, it is illegal to operate, manufacture, import, or offer for sale, including advertising (Communications Act of 1934).Blocking radio communications in public can carry fines of up to $11,000 or imprisonment of up to one year.

For jammer used in jail,you may find more information here:
http://www.jammerbuy.com/3g-gsm-jammers/mini-wireless-cell-phone-signal-jammer-cell-phone-blocker-portable.html