The following info have been gathered from various souces on the web
XT Bikes the 'X' stands for CROSS, and the 'T' for TRAIL

Yamaha's first single-cylinder 4-stroke motorcycle made its debut at the 21st Tokyo Motor show in 1975. As well as being the latest in a series of trail (off-road) models Yamaha had introduced at each of the Tokyo Motor Shows since the DT-1's debut in 1968, the new XT500 represented a whole new concept in trail bike design.

The Yamaha DT-1 had introduced many riders to the world of trail riding, and this ground-breaking model was later followed by the introduction of many other trail bikes from other manufacturers. The new Yamaha XT500 was a natural extension of the growing trail bike category, and although it shared the same roots as its 2-stroke brother, the DT-1, it was an entirely different breed of motorcycle.
Powered by Yamaha's first 4-stroke single-cylinder engine, the new XT boasted a large capacity of 500cc, and this was at a time when the popularity of big-single machines was considered by many to be past its peak. During the mid-1970s Britain's BSA was one of the few manufacturers still offering big single machines, but even this renowned brand had not released a new big single model in a good number of years.
Although some bikers preferred the feel of the vertical vibration produced by these big singles to the character of the newer multi-cylinder models, many riders felt that the big single was a crude, uncomfortable and dated design.
Yamaha's brave decision to build a big single was inspired by a request from the product planners at Yamaha USA. They believed that there was a demand for a bike with the power to let you ride at will across the great open spaces of the American off-road countryside. The machine would need to be rugged and simple, and so it was agreed that a big single was the way ahead. And, making it a 4-stroke would also fit into Yamaha's policy at the time of strengthening the quality and range of its 4-stroke engine line.
The project got under way with the clear design aims of building a big single engine that was lightweight, compact, highly durable and beautiful to look at. By now there was also the idea that this engine could form the basis of a road sports model. While the engine design was under way, work then began on an all-new chassis with the strength to take the vibration of the big single, and one that could handle the tough all-terrain riding that a big off-road machine would be subjected to.
With a motto of "a yen for every gram," the development team tried every means possible to keep the new XT500's weight as low as possible, and this was achieved by making the TT500 Enduro machine's crankcase cover out of magnesium, and using aluminium for the fuel tank on both the XT and TT models.
To help create a slimmer machine, a newly-designed semi-cradle frame was used which featured an integral oil tank, the first time this had been seen on a Japanese motorcycle.
Yamaha launched the TT500 Enduro machine without lights in 1975, and the street-legal XT500 followed in '76. They were an instant success in the American market, where they were used mainly as recreational bikes for weekend off-road riding. With the machine's image enhanced by news of the XT500's string of enduro wins in the USA, the XT500 was released in Japan in February of 1976, and became the bike of choice for a young generation of touring riders.
The big surprise, however, was how these models eventually caught the imagination of riders in the European market. After winning the first two places in the inaugural Paris-Dakar Rally in 1979, and then the top four places in the 1980 event, a new generation of fans with a passion for adventure motorcycling was born in countries like France and Germany. The Yamaha T�n�r� brand that grew out of the XT500 would become synonymous with the word adventure among European motorcycle fans in the years to come, and Yamaha would become market leader in the Dakar-style adventure sport market sector.
Released in 1978, and powered by an XT-based engine, the SR500 and SR400 road sports models became strong sellers in many markets, particularly in Japan, where their popularity continues to this day.
Needless to say, the record shows the amazing reliability of this Yamaha big single engine, which the original designers proudly describe today as one with a good basic design that has undoubtedly stood the test of time. 30 years on from its launch, the XT500 and the SR500/SR400 models can justifiably be described as classics that have earned their place in the history of motorcycling.
Yamaha started producing its first 4-stroke enduro in 1976 and it was named XT500C.This bike started the rich heritage for the next generation of reliable enduros. In 1977 the bike became brown and to mark its evolution, Yamaha added a “D” to the name of XT500. It had the same technical features, wheels and structure. As you probably guest, in the following year, the bike was called XT500E. The U.S. model sported the first polished aluminum tank. I should also mention the addition of black fork gators in 1978.The 1979 XT500F model added new rubber turn signals and a brand new kick start indicator. In 1980, the bike was not improved and 1981 XT500H was the final production version of the XT500 in the United States.At the European models we notice the new skid plate and head pipe. The European 1978 XT500E had a new red and white color scheme in stead of a polished tank and black fork gators that were also added that year.1981-1985 XT500H had a new rear fender and headlight. Again, there was little change for the XT from ’86-’89 but we should notice the newer style rims.

When it was introduced on the market, the XT500 had no competition because of its unique 4-stroke engine and reliability. Then, Kawasaki started producing the KX500 and Honda, the CR500. Suzuki also came with DR500 in 1981, and these are the bikes that created the first enduro generation.

The Yamaha XT500 eventually evolved into the XT600 Tenere during 1993 which in turn in 1988, fell victim to disastrous styling changes, introduced to counter the hordes of copycat desert raiders jostling for Yamaha’s market share. A windscreen was added, bolted onto a new, blocky tank. Though the tank graphics were slightly improved, the traditional trail bike side panels were replaced by bland plastic rectangles. The addition of twin headlights and a disc break at the rear brought increased maintenance with no benefits. Whilst the windscreen and a conventional front mudguard undoubtedly increased the bike’s road going potential, the other additions brought cost cutting elsewhere. Conventional chrome-plated wheels replaced the gold aluminium of earlier machines, and the kick start was removed. It isn’t possible to bolt the new front mudguard to an earlier machine, as it attaches to brackets on a redesigned fork leg casting. The old mudguard looked better, even if it caused the bike to behave like a hang glider at speed.
The last Yamaha model released before the current production XT660X and XT660R was the TTR600 which was a street legal enduro
Finally in 2004 came the XT660X and R models which became immediately popular for their look and their reliability and also for being the first Japanese production Supermoto away from the already well known European built bikes such as KTM Usaberg, Husqvarna and CCM
The XT660 was a success straight from the launch date, thanks to their reliable engine, the low maintenance and service intervals which allowed the owners to use this bike as a daily commuter or for wekkend fun.
Powered by a 4 stroke single cylinder, fuel injected engine and coupled with excellent brakes, Excel rims, this bike offers a comfortable and very accurate ride, point and click, perfect cornering and stability in all road conditions. All this in one package that never stop to amuse the rider, be it a quick stroll to the shop or a fast paced ride back home from work this bike is guaranteed to leave a permanent evil grin at every ride.