IBM's website is the best source for info, but here is the short version:
A Series - Now discontinued. This is a desktop replacement machine. It's heavy (7 lbs class), but when IBM still sold it, it had dual drive bays, the fastest Pentium 4-M CPUs, the best GPUs available, and the best FlexView LCD displays.
R Series - The R series used to serve the sole purpose of IBM's mainstream consumer notebook. R series machines don't use fancy build materials, and are a bit heavier than T and X series machines, but they are well built. The R50p series is now IBM's heavyweight desktop replacement (replacing the A31p). The older R40 is targeted at consumers who don't need to drag their machine everywhere. All R series have a single optical bay. Most R series seem to carry a 1-year warranty.
T Series - IBM's corporate thin-and-light. T series machines are made with more advanced materials than R series (titanium & magnesium cases). T series machines have all the benefits of R series machines, but are generally 15% or so lighter and generally carry longer 3-year warranties.
X Series - IBM's ultralights. Don't need a big screen, GPU, or optical bay? The X series is for you. X series machines use advanced materials in their cases. X series machines have media stations that they can plug into for optical bays and aditional ports.
G Series - Big, heavy, desktop replacement with few of the benefits of the R50p or A31p. G series use desktop Pentium 4 processors and Intel Integrated Graphics. It might be good for a stationary office machine but I woudn't want to lug it around.
Originally posted by: ojai00
Can anyone tell me what the difference between the A, G, T, R, and X series are (if any)? I'm looking to get rid of my heavy Inspiron 8000 in favor of a Thinkpad but don't know which one to get. Thanks in advance for any insight.