Johnny is partly correct. This may be more than you were looking for, but here's a quick run down:"J" is simply Toyota's platform code for Land Cruisers. Every Land Cruiser has a "J" in the model designation. It may have originated with their copies of WWII Jeeps, but as far as I know, that's never been an official explanation.
The first part of the model designation denotes the motor. "F" is for an in-line 6-cylinder gasoline engine. There were many variations to the "F" series motors over time, generally referred to as "early F", "F", "2F", "3FE" (the E denotes fuel injection). There were also 3F motors in countries outside the US. In 1993, the in-line 6-cylinder engine was changed substantially and called the 1FZ-FE, for which the model designation became "FZ". That was followed by the 2UZ-FE motor in 1998, which has a "UZ" designation."H" and "B" are diesel motor designations. Similarly, there were different motors over time, most popularly, the 2H and 3B. A 1978 BJ40 would look identical to a 1978 FJ40, except that it would have a 3B engine instead of a 2F.
All of the "J4" series Land Cruisers have basically identical front ends (with regard to body styling), but will have different configurations for the rear. Johnny gave a decent summary above. 40, 41, 42, 43, 44, 45, 46, 47.
Cruiser FAQ:
http://www.cruiserfaq.com/00-01.php
Body styles:
http://www.cruiserfaq.com/04-00.php
Engines:
http://www.cruiserfaq.com/05-04.php