There are 4 types of 7th chords - Major 7th, Minor 7th, Dominant 7th and Minor-Major 7th(this only happens in a minor key).
Major 7 = Notation - maj7, M7. Used in jazz a lot. Half step shy of the octave. Happens on Major chords.
Minor 7 = Whole step shy of the octave. Happens on minor chords.
What are modes? - Just a different whole/half sequence in a key. This video explained it really well.
Minor scale - Take down the 3rd, 6th and 7th notes of the major key by a half step.
Every major key has a corresponding minor key.
The sixth note of a Major scale is its relative minor scale.

Today I learnt