The circle of fifths  is the relationship among the 2 tones of the chromatic scale, the associated major and minor keys, and their corresponding key signatures. The term fifth refers to the intervals between the notes, rather then listing the notes in order.

It goes though all 12 notes but instead of going in order, they are separated by intervals of a fifth. The first note starts with C at the top. then after c is a note separated by a fifth, G (A fifth is seven half steps). then 5 half steps higher then G is D. then the rest are A, E, B, F#, Db, Ab, Eb, Bb, F and then it goes back to C. In Minor They Go from the top A like C has no sharps or flats. then E (1#), B (2#) , F# (3#), C# (4#). Then in G# there are 7b or 5#, for D# there are 6# and 6b and Bb there are 5b and 7#. Then F (4b), C (3b), G (2b), D (1b). The Minors also go along with the key signatures of the majors. C maj0r and A minor no sharps or flats, G major and E minor 1 sharp, D major and B minor 2 sharps are some examples.

Each of these notes represents key signatures. This can help when writing music to show which keys go well together. All majors keys have 7 notes. If you are writing a piece of music and you are in the key of C, you can change to a G major and it will not be difficult to change  because they are similar to the number of sharps in their key signatures. It is more difficult when there is a key change to a key with a larger amount of new notes because they will not go as well with the other notes from the old key because there is less notes in common.