Basically, I view it like this: within a scale (such as the major scale), there exist other scales by simply starting on a different note. For example, C Major is a scale that has the notes C D E F G A B C, right? But, if you start on the second note of the major scale and play up or down to that same note for an octave, you have a new sound or "mode." For example, let's start at the second note of C major, which is D and play up an octave; that is, D E F G A B C D. This succession of notes is called Dorian Minor.
Further, every Dorian Minor will have the same construction of Whole and Half steps. For example:
D to E is a Whole step,
E to F is a Half step,
F to G is a Whole step,
G to A is a Whole step,
A to B is a Whole step,
B to C is a Half Step,
C to D is a Whole step.
So, you end up with a construction pattern of W H W W W H W.
It's easier to envision the whole and half steps on a keyboard. Take a look:
Practice playing every Dorian Minor scale/mode. The reason for this is, (1) to encourage you to learn and practice every Dorian Minor, and (2) to emphasize that they all have exactly the same construction. In many cases, I use the exact same fingering, but just shifting it down or up the neck of the bass. On a side note, by learning all the major scales, you are really learning a lot more scales than you think. For example, if you know the notes to every major scale, then all you have to do to start playing the modes is start on a different note and center the sound of the music with that note. Music that hinges on modes is called "modal," and one of the most popular uses of this modal vibe can be heard on Miles Davis' Kind of Blue. Check out "So What" (D Dorian & Eb Dorian). Check out the iTunes link:

Following in this discourse on modes, I will continue [re]posting the rest of the modes relating to the major scale. That is, the third note of the major scale is also another mode called Phrygian, the fourth note of the major scale is another called Lydian, and so on. Modes are very interesting and have there own unique character. They open up a lot of new musical possibilities, and have helped me continue in the beautiful exploration of this thing called life.
Thanks for reading, and please feel free to ask questions or offer suggestions.

