Good call on the wiki:
BMW Motorrad motorcycles are categorized into product families, and each family is assigned a different letter prefix. The current families are:
C series – Maxi-scooters called Urban Mobility Vehicles by BMW
F series – parallel-twin engines of 798 cc capacity, featuring either chain or belt drive. Models are F650GS, F700GS, F800GS, F800R, F800S, F800GT and F800ST.
G series – single-cylinder engines of 449 to 652 cc capacity featuring chain drive. Models are G450X (now discontinued), G650GS, G650 Sertão, G650 Xmoto, G650 Xchallenge and G650 Xcountry. The 450 cc engines are manufactured by Kymco in Taiwan. The 2009 and 2010 650 cc engine parts were manufactured by Rotax in Austria, with the engine being assembled by Loncin Holdings, Ltd in China.
R series – twin-cylinder boxer engines of 1,170 cc capacity featuring shaft drive. Models are R1200GS, R1200R, R1200RT, R1200S and R nineT.
K series – four-cylinder engines of 1,157 to 1,649 cc capacity featuring shaft drive. Models are K1200LT, K1300GT, K1300R and K1300S. In 2011, BMW Motorrad launched the six-cylinder 1,649 cc K1600GT and K1600GTL.
S1000RR – sport bike with transverse-mounted, 999 cc inline-four engine.
Just curious, do you know makes the G650GS Sertão better for off road than the other models? I've seen that in a couple places, but it's hard for me to tell what's different about it other than the accessories. There's a nice one local to me that I think I'd like to go take a look at.