Cruiser bicycles are a somewhat lesser known breed of bicycles but seem to be making a comeback in popularity these days. Cruisers come in two flavors: the classic cruiser and the BMX cruiser.
The classic cruiser is easily identified by its retro 1950's look. Curved toptubes and downtubes (the two long tubes that makeup the main triangle of the bike frame) and wide, heavily swept back handlebars give the bike smooth lines; a glossy paint job with racing stripes and you've got a pretty slick ride. I believe it was the Elgin Bluebird in the late 1930's that made these bikes so popular (since Elgins were sold through Sears) but it was Schwinn that really glorified the cruiser by producing tons of after-market upgrades. These bikes are built for speed, stability, and comfort, not to mention style, and often come equipped with front and rear fenders and wide padded seats, rolling on 26" and, less commonly, 24" wheels with tire widths of 2" or greater. A high stem that raises the handlebars and a toptube shorter than found on mountain bikes (MTBs) allow the rider to sit up straight more or less unlike the hunchover seen on road bikes or the halfcrouch for BMX and MTB. I see professors sometimes riding a cruiser in a tweed jacket, trimmed white beard and wiry glasses, and damn do they look respectable.
The BMX cruiser is essentially a slightly larger BMX bike. The BMX cruiser is generally used for racing and has a slightly longer toptube, but keeps the basic geometry of the standard 20" BMX. The main differences between the standard 20" BMX and the BMX cruiser are the wheels, the fork, and the handlebars. Obviously, a cruiser has 24" wheels, usually the same width as 20", about 1.75" to 1.95". These wheels are built up from a 36 hole hub with 3/8" axles (though if you're builing a cruiser for street riding and tricks, you can build on a 36 hole, 14mm hub). A 24" fork is used though since your selection is rather limited in this size, you may find it possible to run a 26" fork. Standard BMX handlebars have about 7" to 8" of rise (height) and a width ranging anywhere from 28" to as little as 23" and shorter if you take a hacksaw to it. Cruiser bars on the other hand only rise about 5" and tend to run on the wide end though they're subjectable to custom trimming. Another option is to use a MTB stem and flat bars. It's really a matter of personal preference since the same heights, angles and offsets can be achievd with either setup.
Classic cruisers can be purchased at your local bike dealer, Wal-mart, or any number of online outlets since even traditional race companies like Felt are even selling classics now. BMX cruisers for racing are also readily available at high-end or specialized shops. Of course, you can always build it yourself or buy the components individually and have a shop build it up for you, the advantage of which is learning a lot about the workings of a bike and a tailored fit but at a greater monetary cost.