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In 1876 Bell invented the telephone. In 1893 Tesla demonstrated wireless communication, and in 1897 the wireless patent was issued to him. However, Marconi was generally accredited with the invention of Radio, and received the Nobel prize in Physics in 1909. Fessenden made The first radio voice broadcast from NYC in 1906 . Put all these together, you have a radio telephone, you talk and listen wirelessly. In modern day terms, the radio phone is commonly known as the mobile phone, or cell phone or even the PCS phone.

So what is the difference between a cell phone and a CB radio or a walkie-talkie ?
With a cell phone you can talk and listen at the same time. With the other two, only one person can talk at one time, but NOT at the same time.

Then things get complicated, since there are two types of MODE ( analog and digital), possibly utilizing 4 frequencies. On top of that, there are at least three different networks ( GSM, CDMA and TDMA. The ways these things work or the meanings of which are left as an exercise for the readers.)

The cell phone is actually a sophisticated machine, performing about 1 million operations per second, most converting digital( D) signal to analog (A) signal to listen, and A to D when talking.

So why is this " radio phone" called a cell phone ( short for cellular phone) ?
It turns out that a certain frequency band is used by many users at the same time. In order to share the frequency band, the area of communication is divided into many cells, about 10 square miles each. Within each cell, there is a base station with relatively low transmission power. When the user leaves the cell, he/she picks up the signal from the neighboring cell, hence, the CELL PHONE.

The personal communication services, PCS, such as the Sprint network, has even smaller cells, using digital technology and frequency band of 1900 MHz, and possibly 800 MHz.

Now you may not care about all these, all you want is to pick up a phone and call. It would be even better to communicate world-wide. Right ? Well, it can be done, all you need is $$$.

Get a quad-band [1] ( 4 frequency bands, 850, 900, 1800 and 1900 MHz) GSM phone, and you can call from just about anywhere (Europe, Asia , and North America).You may be able to get by with a tri-band GSM [1]. In some Asian countries, such as Japan and Korea, you need CDMA. You can rent a cell phone on the web [2].

GSM phones have a SIM ( subscriber identification module) card inside, which stores the connection data and the ID number for a given service provider, such as Orange in HK, or T-mobile in north America, NA.You can send and receive e-mail relatively using a Black Berry, BB, or just text messaging from your phone.

Before you leave for a foreign region, subscribe to the provider international roaming service, or get a SIM card for that country (from www.telestial.com). It is cheaper to go with the SIM card.

Or cheaper still, get two phones , one for North America, NA, (850 and 1900 MHz) and one for most of world ( 900 and 1800 MHz), and a SIM card from that country ( not from the web).

That is what I do. I already have a dual-band/tri-mode ( never mind what it is) CDMA phone ( no SIM card inside) for NA network connections, and for roaming in Canada and US. Bought a ~ $300 HKD (HONG Kong dollars, dual-band Motorola C200 GSM phone for HK, China, Taiwan, Singapore, UK, and Europe, don't know about Australia. Will find out if I go to Australia. Get a SIM card in HK ( ~ 60 HKD for 450 minutes of talk time, you need to know someone who knows how to get it at such a discounted price). Now you are all set to visit HK or the four corners of the world, and would never miss a call from someone you'd like to get a call from.

So there you are:
Have phone , will travel.

Reference:
[1] http://www.thetravelinsider.info/roadwarriorcontent/quadbandphones.htm
[2] http://www.rent-mobile-phone.com/america/index.html