Here's the link to an article written here on this board.
http://www.quiltingboard.com/resources/article-72.html
Basically, 1200 yds is the length of thread on the spool.
Size/weight 40 is like saying "dress size 10" only reverse; the higher the number the finer the thread (in general).
A 50 wt thread can be used for piecing and quilting. If I want my quilting to stand out more I use a 40 wt thread.