The major scale is a series of seven notes spaced in intervals of whole and half steps. On the guitar a whole step is two frets. Obviously then the half step is just one fret.
The major scale, which is what most of the music we've ever heard is based on, uses the following intervals... (whole, whole, half, whole, whole, whole, half -or WWHWWWH from now on).
I'll use the key of G major (or G Ionian) as the example since it only has one accidental (F#) and is a pretty common key on the Guitar. The notes are (G-A-BC-D-E-F#) The hashes represent a whole step between two of the notes. So the first half step is from B to C and the second half step would be from F# back around to G; this second G would be an octave higher than the one that starts the scale . These are the notes found in the first mode...IONIAN.
Now, each mode (of the scale) is just the same seven notes but you start from one of the six other scale steps and go from there. So the second mode, DORIAN, would be (A-BC-D-E-F#G-). And the intervals for Dorian are (WHWWWHW). The unique thing about Dorian is the fact that it is a minor scale but has a happier sound than any of the other minor modes. The chord that often follows the key chord in a song is the IV chord, and in Dorian mode the IV chord (D) will be major.
PHRYGIAN would be (BC-D-E-F#G-A-) or in intervals...(HWWWHWW). Now the interesting thing about Phrygian is that it has a flat second, or a half step between scale steps 1 and 2. This gives it a pretty dark sound. Some people associate this mode with Spannish sounding classical guitar. Others may think of it as a metal mode because of the fact that it is in the minor family of scales (Dorian, Phrygian, Aeolian, and Locrian) and is one of only two modes that has a flat second (B to C).
LYDIAN is one of the three major modes but it has a sound that is often described as "melancholy". The reason for this is the sharp fourth step (in relation to Ionian). Here are the notes and intervals (C-D-E-F#G-A-B and WWWHWWH). This is a mode that is seldom used but has a distinct sound. Some examples of music that is in this mode are...Satriani's "Flying In A Blue Dream", Sublime's "Jailhouse", and The Grateful Dead's "Fire On The Mountain". The easiest way to tell if a song is in Lydian is by the two major chords that are a whole step apart (i.e. C maj / D maj)
MIXO-LYDIAN (D-E-F#G-A-BC and WWHWWHW) is a great "jamming" mode. It is the same as Ionian but with a flat 7th scale step. A good example of a Mixo-Lydian song is Van Halen's "Panama". This is the mode that matches up with major blues the best because of the flat 7th step or Dominant chord sound. You can't go wrong if you try playing any combination of the three major chords in this key (D, C, and G) but start with the D chord to get the Mixo-Lydian sound.
AEOLIAN (E-F#G-A-BC-D and WHWWHWW) is the natural minor scale. This is the mode that most minor key songs are based on. Also, this is the mode that matches up with the Minor Pentatonic scale. In addition, Aeolian is what is known as the Relative Minor. Meaning that if you are in G Ionian (Major Key), you can still play the Emin Aeolian scale and maintain the G major sound. Actually you can do that with all of these modes but since you can use Minor Pentatonic here it is a favorite option for many guitarists whether they are beginners, intermediate, or advanced players.
Finally we have the LOCRIAN scale (F#G-A-BC-D-E and HWWHWWW). This one is tricky. Like Phyrgian it has a flat 2nd scale step but it also has a flat 5th scale step. The combination of the two is what makes this the most "dark" sounding mode. The only songs I have ever come across that are based on the Locrian scale have been written by some kind of metal or progressive rock band. The main function of the chord that represents this mode (F#min7b5) is to create tension in the music that is resolved to some other chord, usully to G (Ionian), or Emin (Aeolian). However, when used correctly this chord / mode works similarly to the D7 (Mixo-Lydian) chord; again to create a tension and release situation.
Well, there you have it. All seven modes laid out in plain English! Easy enough to say it, but can you play it? To fully understand the application of all of this I recommend trying out some private lessons with a qualified instructor, or buying some lessons like the ones on http://www.onlinelessonvideos.com . They will ensure that you are getting the most out of your precious time so that you won't have to repeat the same directionless approach over and over again. I know that feeling well because that's what I did for many years before I finally got serious enough to get into the modes and music theory. Good Luck!