They are run by the same organization (Theater Development Fund).
TKTS....is day of sales only, open to anyone as long as you are willing to stand in line and take your chances.
TDF is limited membership, that you pay for annually giving you much better discounts than tkts and you cann get tix in advance...but not often FAR in advance. You do need to qualify for membership, however.
There is a thread far down on this page giving more detail.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.