A turnbuckle is essentially a pair of threaded eye bolts connected with a central body. The end fittings thread into the body frame and are fully adjustable. Turnbuckles are used to provide tension or adjust the length for a given piece of line (may be soft rope or wire rope, or even chain). More than just eye bolts, the end fittings can be in the form of a jaw (aka clevis) or hook, meaning a turnbuckle’s end fittings can be customized to suit your needs. They come in a variety of diameters and lengths. The bigger the diameter, the high the load capacity. The longer they are, the more adjustment they allow you (the amount of adjustment available is referred to as "take up").
You have a choice when you shop for turnbuckles. Depending on your application, you may elect to go with a high-quality domestic product from Crosby or Chicago. When you want the best, go with one of these domestic sources. For other users, economy is the rule. If you want a turnbuckle that is designed to meet the same federal specifications as the domestic manufacturers, yet costs a lot less, then an imported turnbuckle is your best bet. They come in the same sizes, and offer the same end fitting options as the domestic turnbuckles too.