Picking the right fuse can be confusing but once you understand what the various codes mean, you can pick the right fuse for the job. The codes on a fuse relate to two things, the type of base and the degree of time delay of the fuse.
Fuses have two types of bases, the Edison Base (Type T fuses) and the Rejection Base (Type S fuses), also known as a "tamper-proof base".
The Edison Base Type T fuse works with the typical socket used in the fuse box seen in older homes and the fuse base looks like a light bulb base.
A Rejection Base Type S fuse actually consists of two components, an adapter base that screws and locks into the Edison socket in the fuse box, and the fuse itself. Each different amperage rating of a Type S fuse has a matching socket base adapter with a different sized thread that prevents mismatching (rejection base). It stops a person from putting a 20 Amp rated fuse in a 15 Amp circuit.
Putting in higher rated fuses than a circuit was designed to handle was a problem with older homes. Once the wrong amperage fuse is installed you really can't tell it is improper because the wire size is the determinant of circuit capacity. So what happens is that people replace the wrong size fuse with another one since they assumed the blown fuse was correct. This resulted in many circuits being dangerously unprotected. Using a Type S fuse with its rejection base adapter prevents tampering or mistakes from being made.