Scales and modes are a common cause of confusion. To be honest, I'm not surprised that modes cause confusion for guitarists because they are normally taught so badly. It would take a lot more than the length of this article to give an in depth lesson on modes so that won't be happening right now. However, what I will discuss briefly is a few details about the difference between a scale and a mode. Hopefully this will at least attempt to clear up some of the confusion.
There are two main causes of misunderstanding with the subject of modes. The first one is that you might have trouble seeing the point of them. If they use the exact same notes as another scale then they cannot possibly be something different. The only time this makes sense is if the scale or mode is played in perfect sequence, starting and ending on its root note. Well, I don't know about you but that's certainly not how I use a scale in a solo or melodic phrase, or rarely at least. If that is your point of confusion, let me set the record straight for you. You are absolutely right to be confused and if a guitar teacher tries to tell you otherwise, rest assured, the teacher is the one who is wrong. The start and end point of the scale only describes the notes relationship within the mode, the mode's sequence of notes, nothing else. This kind of teaching will teach you nothing whatsoever about the use of modes.
The second problem, again, another myth perpetuated by poor teaching, is this. Modes are not used for playing over chords in the same key. For instance, in the key of C major we have the chords Cmaj, Dmin, Emin, Fmaj, Gmaj, Amin, Bdim. Each of these chords in the key of C can be said to be linked to each of the modes derived from the C major scale. Makes sense, and it does hold some truth. Unfortunately though, it still has nothing to do with using modes, modally. A lot of teachers will tell you that you can play the mode of the corresponding chord when you create a guitar solo. I.e., C major scale over the C chord, D Dorian over the D minor chord etc. Sure you can do this, but when the chords in the piece of music all belong to one key, you still aren't playing modally, so teaching it this way is pointless because it serves no purpose.
A scale is a scale regardless of it's start and end notes. What determines a scale is the intervallic distance between the sequence of notes. For example, the C major scale and the D dorian mode share the same notes. In C major the notes are C D E F G A B C in that order and D Dorian is D E F G A B C D. The only time they are different is when they are played, in isolation, in the order they are written.
If you play a D Dorian scale over a piece of music with a tonal centre in the key of C major, then you are still playing C major, not D Dorian. The key is what defines the tone. This is where things get complicated and is very difficult to explain if you have limited knowledge of music theory. The easy rule to remember is this. If you are playing a particular mode then the underlying music, or tonal centre of that piece of music also has to match that of the mode. Therefore if you play a chord sequence that is clearly in the key of C, i.e., C Am F G then it doesn't matter whether you use C major, D Dorian, F Lydian or G Mixolydian, it will all be in C major whether you like it or not.
Modes are a complicated subject. If you want to learn about them and their uses properly then you are going to find a lot of misinformation out there. The best product I have found so far that does the best job of showing you how to use modes properly is Frank Gambale's Modes No More Mystery. Not only does this video do a great job of showing how modes really should be used, it also is one of the most inspiring guitar tutorials I have ever owned. Franks guitar playing is so good that even once you understand how to use the modes, you'll find yourself regularly firing up this video just to play along with it and watch as he plays through some truly awe inspiring solos effortlessly. If I could play guitar half as good as Frank I don't think I'd have any goals left, the man is pure genius.