A crescent wrench looks a lot like a monkey wrench; in fact, most of the simple adjustable wrenches you know look like crescent wrenches. A crescent wrench is usually made of steel and has a relatively flat handle that's a number of inches long. At the end of the handle are jaws, one of which is fixed and the other of which moves when you turn the worm gear (or slide adjuster) right below it. On a crescent wrench, the jaws are nearly parallel to the handle. In contrast, a monkey wrench's jaws are perpendicular to the handle. A crescent wrench is useful for turning nuts and bolts within a certain range of sizes.
However, the term "crescent wrench" is a bit misleading. A Crescent wrench is a name-brand product that dates back to the early 1900s. It became popular enough that people started calling all adjustable wrenches by the name crescent wrench. The original Crescent wrench was developed by an inventor from Jamestown, New York, named Karl Peterson. He reportedly got the idea from a Swedish visitor who described an adjustable wrench to him. Peterson created a wooden version, and then worked to create a metal version. Peterson was already advertising a commercial model by 1910. Cooper Industries purchased the Crescent Tool Company later on down the line.
Early versions of Crescent wrenches look more or less like today's models, but there was also once a double-ended Crescent wrench that is no longer made. That type of Crescent wrench had jaws at both ends -- one big and one small. The other major change to Crescent wrenches is that once, all versions had a worm gear, which was kind of like a little barrel you turned to adjust the wrench. Now, many Crescent wrenches come with a slide gear that lets you adjust the size of the wrench's opening by moving a slider up and down.