Number of out for straight-flush

This is a total newbie question.... What is a "straight vs flush draw" exactly, and why does it have 6 outs?

Not sure what you're really asking here. Open ended straight draws have 8 outs and gut shot draws have 4. Flush draws have 9 outs.
With the str draws and flush draws you sometime have to discount some outs as certain cards can help your opponent more then you.
So if you're on a str 8(outs) draw and your opponent is on a flush draw you have to subtract two outs from your draw-the 2 outs that will give your opponent his flush. Hence you have a "str vs flush draw" with only 6 outs.

Ex:

Your cards:
Opp cards:

Board card:

You have 8 outs to make a str-four 3's and four 8's but if the 3 or 8 of hearts come up you're opp gets his flush and beats you. So you really have only 6 out to make your str. 8outs-(3h,8h)= 6 outs.

Hope that helps.

Along the same lines of discounting outs to your opponent (which you can really only do in an all-in situation when you can see his cards), you also discount outs that could make two different hands for yourself, i.e. you don't count the same outs again to make a different hand.

For example, if you had 4567 to the straight, you'd have 8 outs (four 3's and four 8's) to make the straight. If you held a 4 flush draw in hearts, you'd have 9 outs to make the flush. But if you held both, you'd deduct 3h and 8h from your flush outs (or alternatively, from your straight outs, doesn't matter as long as duplicate cards are excluded). So instead of 17 total outs you'd have 15.

Wow, great input guys.. I actually never even thought about it... well I probably have but just haven't realized that is how it works.