Today's models have a battery-powered electric motor that drives the car at slower speeds.

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What is it?

There are several kinds of hybrids. In general, today's models have a battery-powered electric motor that drives the car at slower speeds and a gas engine that kicks in at higher speeds. The engine also helps recharge the battery, along with energy captured from the rotation of the wheels during deceleration.

What's good about it?

Hybrid mileage can be 25 to 30 percent higher than that of ordinary gas-powered cars. That's because the electric motor, which requires no gas, does most of the work during driving conditions where the gas engine is least efficient—like stop-and-go urban driving. Since they burn less fuel, hybrids emit fewer greenhouse gases, too.

What's bad about it?

Big battery packs and other components can add $2,000 or more to the cost, compared with conventional models, and it can take years to pay off the premium through lower gas bills. The batteries, usually lodged in the rear cargo area, also reduce storage space. And the batteries in today's hybrids require nickel, which is costly.

Where would it be most useful?

They're great for people who take lots of short trips or drive frequently in traffic. At highway speeds, there's less of a payoff, since the gas engine does most of the work. Hybrids perform better in warm climates than in cold ones, since low temps can degrade battery effectiveness.

How much will it cost?

They can be pricey. At about $23,000, for instance, the Honda Civic hybrid is roughly $4,000 more than a basic conventional model. The Lexus RX400h, at nearly $42,000, tops the comparable RX350 by about $5,000. But there's always some payback from lower gas bills, fewer refueling stops, and, in some cases, tax breaks.

When's it coming?

By the end of 2008, there will be nearly two dozen hybrids on the market, including sedans, crossovers, luxury makes, full-size SUVs, and pickup trucks.

What's taking so long?

They're available now. The next step will be "dual-mode" hybrids that have two electric motors, are more efficient, and are more suitable for big SUVs and pickups that have four-wheel drive and heavy towing capability. The first dual-mode hybrids are the Chevy Tahoe and GMC Yukon SUVs.

Who's doing it?

Toyota and Honda dominated the hybrid market for several years, but now virtually all manufacturers plan to offer a lineup of hybrids.

Could it be a silver bullet?

Probably not. Today's hybrids still require a gasoline-fed engine and two drive systems, which is less efficient than a single power source. But hybrids are helping drive up fuel economy, propel battery technology, and make consumers comfortable with newfangled cars.