As I am sure you know, there are zillions of kinds of fashion fabrics out there. In this guide, I am just going to focus on fashion knits, which is where I concentrate my research and buying for our store, Lucysfabrics. 

First of all, when buying knit fabrics be sure that you are buying fabric appropriate for the application. If you are sewing activewear such as a swimsuit, dance costume, or leotard, be sure that the fabric is intended for activewear. It should be sturdy, and have good stretch and recovery. It should have spandex in it (Lycra is a brand name for the elastic fiber, spandex). With the exception of sweats or hoodies, activewear fabric should dry out quickly when wet. Some activewear fabrics can also be used for fashion if the pattern calls for a stretchy knit, but knits intended for fashion only do not work for activewear -- even if they do have spandex in them. Of course, if you are making an accessory such as a swimsuit coverup or flowing skirt for a dance costume, you have a lot more leeway -- just be sure it can be laundered as you would like.

Here are some knits that are essential for fashion sewing. These are my own definitions, (except where otherwise noted) so I am not sure how scholarly this is, but this is how I find it easy to differentiate them. Customers often ask about -

Fleece - Fleece, the #1 best selling fabric in the US, is most commonly seen in fabric stores as printed or solid, 100% polyester, brushed on front and back, and usually used for blankets or outerwear. This differs from fleece used for sweats, which is brushed on the back only, usually made of cotton-poly, and has some give. Only "stretch fleece," with spandex, is stretchy.

Matte Jersey - This term is used for a lot of different non-activewear knits, from 100% Rayon (not machine washable) to 100% Polyester. It's often used for evening wear but is very versatile - it can be sexy and clingy, but does not have to be. It's a smooth, non-shiny, moderately stretchy (usually 2-way) fabric that is wrinkle-resistant and fast-drying.

Ottoman - Ottoman is ribbed but flat on the back, kind of like corduroy. It often used in activewear. The less shiny Ottomans also make nice casual wear. It can be 4-way stretch or 2-way stretch.

Rayon Jersey - Rayon (or viscose) Jersey is, as the name implies, a t-shirt type fabric (not an activewear fabric) made of Rayon-Spandex. It drapes and prints more nicely than cotton, but like all rayons, it can be damaged by handling it when wet, and by using excessive heat. So, hand wash or use the gentle cycle on your machine, and flat dry, and you will enjoy your garment for years. France, Italy and Germany are the main producers of the high-end, printed version. I have seen less expensive solids lately, but they are not as sturdy or drapey.  

Rib Knits - Ribbed knits are knittted with ribs -all the way through- and are very stretchy in the direction perpendicular to the rib. The number of the rib defines the size of the rib and their spacing, like 1X1 (baby rib), 1X2, etc. They are used often in t-shirts and loungewear. They can be cotton or a blend of synthetic fibers and come in a variety of different weights.

Slinky - Like Matte Jersey, this term is sort of hard to define but you know it when you see it! Basically it is an acetate-lycra fabric with ribs that is very stretchy and is most famous for being almost completely wrinkle-proof. Usually it is somewhat shiny, but it can be crepey or even printed. All but the lightest colors are opaque. Chicos' "Travelers" collection is an example of high-end slinky.

Yarn-Dye - Yarn-dye means that the yarn was dyed before the fabric was knitted or woven. In the case of stripes, yarn-dye can be preferable to printing, because the fabric looks almost the same on both sides and the back of the fabric (it does look more finished on the right side, however). And, the back of the fabric won't bleed through the stripe when stretched.

Linings - Finding linings that will work for knits can be difficult. Most fabric stores only carry the woven types used for jackets.  You can double your fabric, or use an inexpensive, thin solid color fabric as a lining. However, we carry some linings especially for knits:

• 70D Lining: If you look at ready-to-wear skirts, dresses and crocheted items, the most common stretchy lining you will see is 70D (70 denier) polyester knit lining. It is inexpensive and has 2-way stretch. It can be washed to death. Since it's so inexpensive and easy to handle, it also makes a good "muslin." 
• Heleneca Nylon Lining: For bras and activewear, the most common lining is Helenca nylon lining. It's 100% nylon but it's knitted in a way that it gives it 4-way stretch. It has a breatheable, meshy texture, and sometimes people double it. Helenca comes in a lot of different weights. After some experimenting, I think I have found the best weight for a variety of uses which is still easy to handle. When too thin, it can be snaggy and tough to handle. Regardless, it does need to be handled with care.
• Anti-Static Polyester Stretch Lining: This is actually a woven lining, but it can work for knits also. It is a smooth, matte, 100% polyester lining with a little bit of stretch (not as much as the two listed above). Knits won't stick to it.

And finally..... bit about caring for your knits: All knits will snag or run, some more easily than others. Many knits may be machine washed, but I suggest washing the more delicate ones like matte jersey in a mesh lingerie bag. Don't wash your knits with bras, clothing that has zippers, or any other hardware that might snag.  I use a gentle soap and cold water, and I dry my knits flat on a drying rack to prevent them from getting stretched out.