UNDER the glass counter top at an electronics shop in midtown Manhattan, a shopper surveyed two dozen hand-held cellular phones, sleek and bristling with buttons and controls, looking not so much like Maxwell Smart's shoe phone as like Captain Kirk's communicator.

"What is the difference between this $40 phone and this $90 phone?" asked the shopper, a man of about 60 who wanted to take the plunge into the brave new world of "personal communications."

"The power," the salesman replied. "The $90 phone is a lot more powerful. It also has digital sound."

The shopper, perhaps wary of the details, had brought his son with him. "Can I reach this phone anywhere?" the son asked.

"Anywhere," the salesman said. "It will ring anywhere."

Neither of the salesman's answers was correct. Experts in cellular technology say that customers who don't know what they are buying often do business with salespeople who don't know much about what they are selling. Besides, what determines the cost isn't the hardware. These two shoppers never asked what may be the most important financial question: Which company would provide the service for the phone?

Cellular phone companies have set up thousands of antennas all around cities and big towns. Each antenna is at the heart of a "cell," which in a city can cover a few square blocks and in rural areas can be many square miles. The phone sends and receives signals with a single antenna, which then routes the call over the conventional wired telephone network to anywhere in the world. If the user travels from one cell to another, the call is automatically rerouted through a different antenna. The phone tells the cellular system where it is so it can receive calls.

"Power" is moot when comparing hand-held phones, according to the two New York-area cellular phone service carriers, Cellular One and Nynex Mobile Communication, and Motorola Cellular, a manufacturer.

The Federal Communications Commission limits hand-held phones to six-tenths of a watt, but they usually operate at less than that, not because they are weak but because the cellular system tells them to. If the antenna can hear the caller clearly, it exercises something called dynamic power control, telling the phone to turn down its electronic volume to the minimum level needed, so that its signal does not interfere with other cellular phones using the same frequency in a neighboring cell. The process is rather like the host at a cocktail party who tells each knot of guests to keep their voices low enough so that others can also carry on a conversation.

Hand-held phones work well from the sidewalk or from inside cars in cities or in densely settled suburbs, where the cells are small. In rural areas, car phones work better because they operate at up to three watts, or five times the power of hand-held phones.

Another subject of much confusion is the "digital" cellular phone. Some just entering the market transmit in computer code, which is decoded by the phone and turned back into audible sound, in much the way a compact disk stores sound. The result is clearer sound and better range. But nearly all cellular phones on the market use analog signals, as do radios. Sometimes salespeople will claim that a phone uses analog signals but has "digital sound" in its inner workings. Phone company representatives say they do not know what this could mean.

Cellular phones sell at very low prices because companies that provide cellular service want to boost sales. They pay big commissions to the electronics shops, which in turn mark down the phones. Some phones sell for less than a Walkman or even a moderately sophisticated pocket calculator, far less than they cost to make. One model by Motorola Cellular is now offered at $19, with a $50 rebate, meaning that the store pays the customer to take it.

The big cost with cellular phones is not the hardware but the service. "It's like when someone gives you a puppy," said David A. Pinsky, a spokesman for Motorola Cellular. "You have to feed it and give it shots."

Ray P. Dolan, the vice president of marketing at Nynex Mobile Communication, said purchasers are "precommitted to the blades, and getting a cheap price on the razor."

Both Nynex and Cellular One, the other New York service provider, said that new buyers tend to use the phones heavily until the first bill comes in, then reduce their use quickly. Mr. Dolan called the effect of the first bill "sticker shock," since first bills commonly run hundreds of dollars.

Robin Traum, a spokeswoman for Cellular One, said, "They've got a cell phone most likely for the first time, and they will use it to notify people and say, 'Hey, guess where I'm calling from?' "

In the New York area, the two service companies take different approaches. Nynex, Mr. Dolan said, signs up most customers for its "simplicity plan," under which they pay a monthly access charge plus a charge per minute, which varies according to the number of minutes used in the month.

For example, for usage up to 60 minutes a month -- which may sound like a lot but is only two minutes a day -- the Nynex access charge is $29.95, and the rate is 59 cents a minute during peak hours and 30 cents a minute off-peak. (Prices for access and for each minute are lower at higher levels of use.)