The color wheel has earned a place in the hearts of many artists and designers across the globe. A color wheel or color circle is an abstract illustrative organization of color hues around a circle that shows relationships between primary colors, secondary colorrs, complementary colors, etc.
Some sources use the terms color wheel and color circle interchangeably;  however, one term or the other may be more prevalent in certain fields or certain versions as mentioned above. For instance, some reserve the term color wheel for mechanical rotating devices, such as color tops or filter wheels. Others classify various color wheels as color disc, color chart, and color scale varieties.

The wheel's construction is actually quite simple. You have your 6 basic colors: red, orange, yellow, green, blue, and purple. Then, depending on which wheel you're looking at, you have extra, "in-between" colors that are mixes of the basic colors.

There are names for all of these colors, which are important to know. The following is a list of all of the names of colors and what they're good for.

Primary Colors: Red, Yellow, Blue. These 3 colors are the base colors for every other color on the color wheel. This is why they're called "primary." When you mix two primaries together, you get a secondary color.
Also note the triangular positioning of the primary colors on the color wheel, and how the secondary colors are next to them.
Primary colors are useful for designs or art that needs to have a sense of urgency. Primary colors are the most vivid colors when placed next to eachother, which is why you'll notice that most fast food joints use primary colors in their logos, as it evokes speed.

Secondary Colors: Orange, Green, Purple. These 3 colors are what you get when you mix the primary colors together.
They're located in-between the primary colors to indicate what colors they're made from. Notice how green is in-between yellow and blue.
Secondary colors are usually more interesting than primary colors, but they do not evoke speed and urgency.

Tertiary Colors: These are those "in-between" colors like Yellow-Green and Red-Violet. They're made by mixing one primary color and one secondary color together. There can be endless combinations of tertiary colors, depending on how they're mixed.

Complementary Colors: Red and Green, Blue and Orange, Purple and Yellow. These are the colors directly across from eachother on the color wheel. Don't let the name fool you, they rarely look good when used together. They're called "complementary" because, when used together, they become extremely vibrant and have heavy contrast.
Complementary colors are useful when you want to make something stand out. For example, if you use a green background and have a red circle on it, the red will jump off the page and be almost blinding.

Analogous Colors: Red and Orange, Blue and Green, etc. These are colors right next to eachother on the color wheel. They usually match extremely well, but they also create almost no contrast. They're good for very serene-feeling designs and artwork where you want viewers to feel comfortable.