Every BMW motorcycle model name starts with a letter designating the engine family, then a number for the engine displacement, then letters describing the model type.

BMW currently has three engine families:

K "Brick"
A liquid-cooled, inline engine. Originally there were three- and four-cylinder versions; now only the four-cylinder remains. Older bikes with this engine get called "flying bricks". It makes a distinctive whining sound when running.
R "Boxer"
BMW's renowned horizonally opposed air cooled twin started its life as an aircraft engine. After World War I, Germany was banned from producing aircraft, so BMW, started building motorcycles around its existing airplane engines. This heritage should give the reader some idea of the level of stamina and reliability BMW riders expect from their machines. The current R engine is air- and oil-cooled and is available in various states of tune, depending on the application.
F "Thumper"
A liquid cooled single. Unlike the R and K, the F engine and transmission use a shared oil bath and a wet plate clutch. With this layout, the traditional shaft drive layout BMWs have had since 1923 had to be abandoned: F bikes use either a chain or a belt for the final drive.

Current model types include:

Cruiser. Low-slung, grunty, sexily styled, with a laid-back riding position.
Gelandestrasse, meaning roughly "on the land or street." Rugged, enduro-style motorcycles designed to handle both pavement and rough, unimproved roads. The R1150GS has been described as the Swiss Army knife of motorcycles.
The Roadster is BMW's standard.
Rally Sport. A sport touring bike with the emphasis on sport.
Rally Tour. A sport touring bike with the emphasis on motorcycle touring.
Luxury Touring. BMW's K1200LT is a serious competitor to Honda's GoldWing.

BMW motorcycles have an excellent reputation among serious motorcyclists. Their owners tend to be safety-conscious, experienced riders. The most seasoned BMW pilots think nothing of riding two hundred miles to some remote mountain road, dragging the pegs through the twisties (humiliating the wannabes while the wife dozes on the pillion seat), breaking for lunch, then heading back home.