Originally Posted by: CSchlegelLook at the lessons on intervals, scales, chords and (most importantly) the relationship between scales and chords in Guitar Fundamentals 2:
http://www.guitartricks.com/course.php?input=2

After that look at my concise overview of music theory:
http://www.guitartricks.com/tutorial.php?input=495

And improvisation:
http://www.guitartricks.com/tutorial.php?input=483
http://www.guitartricks.com/tutorial.php?input=491

In a nutshell, certain chords belong together (in "families" or "key groups") because they all contain notes that are from one scale (otherwise known as a "key"). :)