The circle of fifths first reveals to us how many sharps or flats each key contains. Starting the count from C major, which has 0 sharps and 0 flats, F major is on the flat side and has 1 flat. Bb has 2 flats. G major (on the sharp side) has 1 sharp, D major has 2 sharps, A major has 3 sharps and so on down to F# or Gb (can be called either name) which has 6 flats or 6 sharps depending upon which name you choose to call it.
The circle also reveals which notes of each key/scale are sharped or flatted. B is the first note to be flatted on the flat side. F is the first note to be sharped on the sharp side. For instance, G major has one sharped note, that note must be F#. F major has one flatted note, and that note is B.
As stated before, the interval between any 2 adjacent notes is a perfect fifth (flat side is down a fifth, sharp side is up a fifth). But it is easier, to always count from C and move down either side, thus if counting from C and moving down the flat side, (left) the notes are in 4ths, while starting from C and moving down the sharp side, (right) the notes are in 5ths. The notes flatted or sharped follow the same order. So the order of the flatted notes is Bb, Eb, Ab Db, Gb and Cb (B) while the order of the sharped notes is F#, C#, G#, D#, A#, E# (F).
Once a side gains a sharped or flatted note, that note is never lost. For instance, on the flat side, F major has one flat, That note is Bb. Bb major has 2 flats, the first being Bb and the second being Eb. The key of Eb has 3 flats the first two being the previous Bb and Eb notes, the third being Ab. The same holds true for the sharp side (see chart below).
If all of this is still confusing, try simply comparing the list of the scales below with the circle of fifths. This may clear some things up.
Here's another web site that also explains the circle of fifths as well as offering some additional great information on scales, chords, and jazz music theory.