Cubic Zirconia, Moissanite, Swarovski Crystals, Austrian Crystals, and Rhinestones
Buying jewelry can be confusing if you don't know the lingo for real gems, synthetic gems, and imitation gems. Real gems come from mines and are cut and polished so they refract and scatter light, what we call sparkling. Bona fide gems have a crystalline structure. The ones most often used in jewelry are diamonds, emeralds, sapphires, rubies, and amethysts (a type of quartz).
Synthetic gems are also crystals, but created in laboratories where manufacturers simulate the temperature and pressure conditions that occur underground to make real gems. Synthetic emeralds and rubies have the same chemical composition as real emeralds and rubies, and almost the same crystalline structure. Only gemological experts know how to tell the difference. Cubic zirconia is a lab-created gem often used in place of diamonds, but it is not the same thing as a synthetic diamond because the two have different chemical compositions. CZ is a crystalized form of zirconium oxide, while diamonds are crystalline carbon. Lab-grown diamonds also exist and are used in both jewelry and industrial cutting tools.
Imitation gems are pieces of glass cut to resemble real gems and coated with different substances to give them the lustre and sparkle of real gems. Glass does not have any crystalline structure, so the term "crystal" imitation jewelry makers use is a misnomer. In fact, glass is an amorphous solid, like an extremely viscous liquid whose flow is imperceptible to the human eye.
For diamond look-alikes, the most commonly used materials are cubic zirconia, moissanite, Swarovski crystals, Austrian crystals, and rhinestones.
As mentioned above, CZ is a lab-created gem of zirconium dioxide (ZrO2) with a cubic crystalline structure. For comparison, sodium chloride table salt (NaCl) is also a cubic crystal. CZ is not synonymous with synthetic diamond, but it is a diamond simulant. Reasons why CZ is a popular diamond alternative are its lower cost and its gemological properties. Real diamonds have a refractive index of 2.42 and dispersion of 0.044, while CZ has a refractive index of 2.15-2.18 and dispersion of 0.058-0.066. These profiles are similar enough to make cubic zirconia one of the best substitutes for real diamonds.
Diamonds are rarely colorless, most often having a slight brown or yellow tinge due to impurities . Cubic zirconia, on the other hand can be made totally colorless, lightly tinted, or intensely hued. Laboratory techniques have advanced so that its even possible to make multicolored CZ gems. The easiest way to tell cubic zirconia from a real diamond is to compare loose gems. CZ is about 62% more dense than diamond. 
Moissanite gemstone is a crystalline form of silicon carbide (SiC). Moissanite is a much rarer mineral compared to diamonds, which are actually quite abundant on Earth but kept in artificially low supply by DeBeers. Because of its color and fire, moissanite is a diamond replacement gaining in popularity. The process for making moissanite crystal into jewelry-grade gems is under a patent that expires in 2015, after which the public will start to see more options for this gem used in jewelry. 
Swarovski is a trademarked brand of cut glass gems and glass figurines; the company is located in Austria. Swarovski "crystals" are made of lead glass, which is sometimes referred to as "crystal glass." It's the same material used to make high-end wine glasses and champagne flutes, but jewelry-grade lead glass contains a much higher lead content, up to 35%. Swarovski crystals have proprietary chemical coatings that give them a rainbow appearance.
They are a popular choice in making large draping necklaces and earrings since real diamonds or CZ of that size would be prohibitively expensive. High-end chandeliers may also have Swarovski crystals to better scatter the light for a dazzling effect. As far as price goes, they are cheaper than CZ, but more expensive than other brands of cut glass gems. 
Austrian Crystal is a generic term for any kind of lead glass gem made in Austria. Swarovski crystals are the most well-known brand, but there are several smaller companies in the same industry. In promotional materials and advertisements you may see the phrase "genuine Austrian crystal." All this means is that the crystals are produced in Austria. The title is a bit silly since lead glass crystal is itself an imitation of genuine gemstones. They come in a wide variety of colors to simulate real gems, for example, green and red as imitations of emeralds and rubies.
Rhinestones are smaller imitation crystals often used in costume jewelry or as accents in pearl jewelry. Originally, rhinestones were pieces of glass or mineral/rock crystals (such as quartz) collected from the Rhine River.
Whether they are made of lead glass or quartz, manufacturers of rhinestones cut them to have facets like real gemstones and apply a metallic coating or shiny foil backing on the underside to make them extra sparkly. Swarovski is one manufacturer of rhinestones.