The term "karat" has been used for centuries to as a unit of gold purity. This is not the same as "carat" which is a measurement of diamond weight. Normally we see 10k, 14k, or 18k jewelry, and naturally the higher you go the more pure the gold is. But what do they really mean?
Let's start with 24k gold, which is pure, 100%, real mccoy gold. From now on, think of every piece of gold as having a purity that ranges between 0 and 24. So, if we have a 10k piece of gold, that means 10 parts of 24 is gold, and the other 14 are other alloys. This equates to 41.67% gold and 58.33% of other alloys. Here is a chart that lists out the other purities.
What other alloys are being used to make up the rest of your jewelry? There are an array of alloys that are typically used, such as silver, copper, nickel, zinc, or palladium. Some metals are chosen over others to alter the color. For example, "rose" gold has parts of copper to give that reddish color, and white gold is mixed with a white metal like silver, nickel, or palladium.