I often get asked which Aurifil thread people should be buying so I thought I'd do a quick run through of which weight of thread is which and what each thread is best suited for. Rule no. 1 is the higher the thread number, the thinner the thread. Rule no. 2 is grab yourself a colour card if you can as each weight of thread comes in each one of Aurifil's 252 colours so you can always find the weight you need in the colour you want.
Aurifil 50wt is the finest (thinnest) of the threads and so is perfect for machine and hand piecing. It can also be used for quilting where you want the threads to melt into the background. It comes on an orange spool. You will find you can piece much more accurately with this thread than with other threads.
Aurifil 40wt is slightly heavier. It can also be used for machine and hand piecing but you will get better precision and the thread will glide more easily through the fabric with the 50wt when hand piecing. This is a great machine quilting thread where you want a bit more definition. It comes on a green spool.
Aurifil 28wt is ideal for machine quilting and should be used when you want the quilting to do the talking. It comes on a grey spool.
Aurifil 12 wt comes in both cotton and wool and is great for hand quilting and embroidery. It will run through the machine if needed too. Both the cotton and the wool 12 wt come on red spools.