In 1989, IBM released the P70 as part of their PS/2 (Personal System/2) line of computers. The P70 was essentially a "luggable", a term describing a computer which requires an external power source to run (there either was no internal battery, or, as in the case with the Macintosh Portable, the battery required additional power from an external power supply to meet the power demands of the system.) These machines typically weigh over 10 pounds, consume a lot of space, and are somewhat as noisy as their desktop counterparts.
The P70 and P75 ('90) were both luggables. IBM's Note series notebooks were early notebook computers manufactured by the IBM, under the PS/2 line as well. Many people give credit to the Note series computers to being a direct influence on the design on the ThinkPad.
Original ThinkPads (1992-2000)
In 1992, IBM split the ThinkPad into its own product line. The ThinkPad differed from IBM and the PS/2 design language; the PS/2 and many other IBM products at the time were known for their basic beige components, typically accented with a off-blue color. The ThinkPad, on the other hand, was black with orange accents.
The original ThinkPad line had three models: the ThinkPad 300, 700, and 700c. These early models shared a common exterior design as the Note series, except for the aforementioned color changes. The model 300, featuring a 25MHz 386 SL microprocessor, weighed 5.9 pounds and was designed by Zenith Data System (ZDS; a division of Zenith at the time) for IBM. This model was the cheaper of the three, with a cost of $2,375 with a standard 80MB HD (or, $2,575 with an additional 120MB hard drive.) The 700 used a 486SLC microprocessor which was standard clocked at 25MHz, but could have the speed doubled to 50MHz. The 700 model weighed nearly a half-pound heavier, and retailed for $2,750 with the standard 80MB hard drive. The 700c, which featured a larger color screen (hence, the "c" at the end) used the same 486SLC processor, clocking in at 25MHz but could be overclocked to 50MHz. This model weighed 7.6 lbs, nearly two pounds heavier than the Model 300. The 700c was the most expensive model at the time, with a price tag of $4,350 with a standard 120MB drive.
As early as 1993, there were ThinkPad systems in space. The ThinkPad 750 flew on a mission to repair the Hubble Space Telescope, after several batteries of tests were ran to ensure they would hold up in space-like conditions. The most popular computer on the International Space Station is the ThinkPad A31p, which was made in the late 1990s before IBM included a trackpad on the ThinkPad. (Navigation was done via the beloved TrackPoint.)
The New Millenium and the Transition
In the early 2000s, the ThinkPad was used to model the ThinkCentre, a desktop business computer made by IBM starting in 2003. They extended the matte black design language (or, the "Think" design language) to the ThinkCentre and even the NetVista and later Aptiva products.
By this time, the ThinkPad was IBM's flagship product. Since IBM was known primarily for business/industrial applications, only their PCs and the Selectric typewriters are "consumer" IBM products. However, IBM's PC Division was considered to be underperforming, as other manufacturers (which started out as PC-clone manufacturers) took most of their market share.
In 2005, IBM sold their Personal Computing Division to Lenovo, a Chinese manufacturer. To this day, Lenovo still sells the ThinkPad and ThinkCentre lines, which are two lines which are flagship products even for Lenovo.
There are two notebook computers which have a huge cult-following surrounding them: the Apple MacBook (anything Apple typically has a cult following), and the ThinkPad. There are people who collect old IBM PCs, and despite primarily collecting PCs and PS/2 models, they typically extend their collections to IBM ThinkPad models. Even the new Lenovo-made ThinkPads have a following, and are known for the reliability and design.
Over 25 years of production, there have been over 60 million ThinkPad computers sold, with over 200 different model configurations. It is one of the oldest continious computer models still produced.