Hexadecimals are a Base-16 System that is commonly used to represent different colors.
This type of color is mainly used for on screen viewing, as it matches up with RGB. Finding a match between CMYK mixes and Hexadecimals can be extremely difficult with vibrant colors. If working with print, stick to CMYK.
While working on a website, or something with on screen viewing, hexadecimals are great!
Within this example, there is the color name, hexadecimal, and then the RGB numbers.
The Hexadecimal form is below the color. Below that is RGB coloring.
Many online tools can convert between hex code and CMYK! Make use of those when needed.