The Duratec is a range of inline four, V6, and V12 engines produced by Ford. In general, any new engine design produced by FoMoCo is badged as a Duratec. This means that in addition to several Duratecs with performance backgrounds, new versions have supplanted the Zetec in the Ford Ka, Mondeo, and Focus. There are several common threads running amongst the Duratec line. Chiefly, there are only three or four different engine blocks in the line. By sleeving or stroking the engines, Ford can utilize the block of a smaller engine for a larger displacement. (For example, my Ford Focus 2.3 L uses the same block as the new 2.0 Duratec, but has a slightly wider bore and a longer stroke)
All Duratec engines utilize fuel injection. With the exception of the smallest engines, all Duratecs are DOHC with four valves per cylinder, although the V6 is capable of using only two valves per cylinder at low speeds. Mazda utilizes modified 2.0 and 2.3 engines in the Mazda3, and 2.3 and 3.0 Duratecs in the Mazda6 (Mazda's engines are badged as MZRs and feature variable valve timing). The turbocharged Ford Focus RS contains the only forced induction Duratec; however, it is disputed as a "true" Duratec, as its cast-iron block and longer stroke lend it more similarities with the Zetec engine range.
The first Duratec was introduced with the European-market Mondeo V6. This 2.5 L powerplant was a relatively economical performer in the Mondeo (a necessity for a European family-car), and was used in the Ford Contour and Mercury Mystique and Cougar*. A modified version of this mill was found in the SVT Contour, producing 200 horsepower. Although not a Duratec itself, the Ford Taurus SHO's V8 was directly descended from this V6. The line now includes 3.0 (featured in the Taurus) and 3.5 liter V6s.
In the last few years, various small-displacement Duratecs have been designed to replace the Zetec line in the Focus, as well as several European compacts. These range from the tiny 1.25 L found in the Fiesta to the Focus ST's beefier 2.3 L. Inclusive are 1.3, 1.4, 1.6, 1.8, and 2.0 liter inline-fours. Most of these engines are newly-designed, with no real connection to the earlier Duratecs. The 2.3 is the exception, as it is relatively unchanged from its use in the Ranger light pickup.
There is a single Duratec exclusive to the Aston Martin marque of FoMoCo. This is the 6.0 L V12, essentially a pair of 3.0 L V6s connected end-to-end. It produces over 400 horsepower, and is found only in the Aston Martin DB7 Vantage and V12 Vanquish.
*Thanks Transitional Man!