There are three material classifications for luggage: hard-sided, semi-soft-sided, and soft-sided. Each has advantages and disadvantages.
Hard-Sided
Hard-sided luggage is by far the most durable. If you are a frequent air-traveler who usually checks your luggage, you should invest in a hard-sided luggage set. Hard-sided suitcases are either molded cases made from plastics or metals or box-constructed cases with metal or wooden frames. Hard-sided pieces are durable and offer significant protection for fragile items. They can be more expensive, however. They are also quite heavy, even before they are packed. Make sure the hard-sided suitcase you are considering has quality wheel construction.
Semi-Soft-Sided
The most popular option, semi-soft-sided suitcases are partially framed, but have soft tops and bottoms. This allows for expansion while still providing protection for the contents. Semi-soft-sided suitcases can be made from real leather, processed leather, natural fabrics (cotton, linen, or canvas), or manufactured fabrics (nylon or vinyl). The material used greatly impacts the cost of the luggage. Leather suitcases can be exorbitant in price, especially if a designer name is attached. Nylon or vinyl cases are usually far less expensive.
Soft-Sided
Soft-sided cases are just as they sound: soft. They are usually constructed with some type of stiffener to help them hold their shape, but they are generally soft, lightweight, and expandable. They do not provide a great deal of protection for the contents inside. Like semi-soft-sided cases, the cost can vary greatly, depending on the construction material.