I am often asked to advise on what sort of interfacing to buy. It really does depend on the fabric you are looking to stabilise or strengthen.
Interfacing comes in many guises, it is either iron on or sew in, comes in different weights (thicknesses) and can now be found in black or white. Many people assume that a lightweight or fine fabric needs a heavier weight interfacing to compensate for the fabric, this is not the case.
A good rule of thumb is to match the weight of the fabric with the weight of the interfacing as much as possible. Although I prefer an iron on (fusible) interfacing, I sometimes buy a sew in interfacing when I know I am not going to get the iron on interfacing to stick to the fabric. The most likely culprits for this problem are wool fabrics or those with lots of loose fibres.
So, now you can buy your interfacing with confidence!