On the turn specifically ...
If we have a gut draw, say holding 98 and the board has T6 (we need a 7), that's 4 outs. We have about 8 or 9% chance of hitting.
If we are open-ended, say holding 98 and the board has T7, that's 8 outs. We have about 15% chance of hitting.
If we have a flush draw, that's 9 outs. We have about 19% chance of hitting.
Also, if we have 1 card to the top of the straight, say 92 and the board is 876, this is not bad, but 1 card to the bottom, say 92 and the board is 345, this is not so good because if we hit the 6 we lose to 7x. That is a lot of combos. 1 card in the middle is okay but not quite as good as top. If we have 92 and the board is T87, if we hit a J we only lose to Q9 (or flush or better of course).
Also, if we have a straight draw and the board has a flush draw, we count our outs a little different. If we have 98 and the board is 76xx with 2 clubs, we'd rather not see the T or 5 of clubs, so personally I count those as half-outs (maybe a better player can comment on this math) because 2 of our 8 outs could potentially kill us. Even worse situation is double-flush board. If we have same 98 on 76xx board but there are 2 clubs AND 2 hearts, this really hurts our outs. We would really rather not see the T of clubs nor hearts, nor the 5 of clubs nor hearts since FOUR of our 8 outs could potentially kill us. So we would tend to fold more here.
With flushes, be careful of weak flushes. Say we have 75 hearts and need 1 more heart on the river. T2 hearts (and many similar
hands), beats us if we hit, they hit too and outkick. Furthermore, if we have a flush already on the turn with 75 hearts, we do NOT want to see a 4th heart on the river. Then any unsuited heart broadway will almost certainly call and beat us. That is a TON of combos. This doesn't mean that we have to fold, but it definitely is a spot to exercise pot control. Flat call a small bet and probably fold to large bet or raise. In fact, in a spot like this it is good to take charge and lead out the turn with a sizing that prices out a 9 out draw.
generally speaking, when we have a non-nut flush draw, we WANT to see big broadway cards in suit hit the board as any card over our top card is 1 less out for Villain. It blocks us from being outkicked. If we have K7 hearts and the A of hearts flops, we now have improved to nut flush draw. So if we have J9s, we want to see A and or K of hearts to improve us and block Villain from outkicking us. Furthermore, if we hit our flush and there is an A and or K on the board, AK, AA, KK and other Ax and Kx hands just might pay us. ; )
Of course just as with any other situation, stacks and Villain tendencies matter. If we are 100bb deep we are more apt to call any reasonable bet, say half-pot might even be worth it, especially if Villain is bluffy or loves to overplay thin value. If we only have 20bb, we probably shouldn't get careless with air and just fold a lot more. This is just another reason why a large stack allows us to chase more to give bad beats.