Thanks to spinayarn99 for asking "in which direction do you spin and then ply?" on my post How can I fix the twist in my 2-ply?.
Most typical spinning is to spin the singles "Z", or clockwise spin on your wheel or spindle and to ply your singles together "S", or counterclockwise spin on your wheel or spindle.
The first cool fact about the letters Z and S is that the bar in the middle of the letter showing you which way your yarn's twist angle is: for yarn spun "Z", the yarn's twist angle is to the "/" direction, like the middle part of the Z; for yarn spun "S", the yarn's twist angle is to the "\" direction, like the middle part of the S.
drawing © The Bellwether, used with permission. From Spindling: The Basics.
Now, that's enough to understand why we call these Z twist and S twist, but there's a second cool fact too -- trace the letter Z in the air. As you start the upper bar, you are moving "->" to the right, or in a clockwise direction, starting at noon and moving to 1 o'clock. Now, Trace the letter S in the air. You guessed it! as you start the upper bar, you are moving "<-" to the left, or in a counter clockwise direction! How sweet is that!
Now, this is just "typical" yarn.
If you cable, typically you do: singles Z, ply S, ply-of-plies (the cable step) Z.
If you spin your singles S -- which you might do if you are just spinning singles, since S twist tends to keep its shape or get a little extra twist in knitting (which is better than losing twist and having your needle split the yarn...) -- this S twisted single is called "widdershins".
I've also seen two singles spun Z and the ply done Z too -- but this is highly active yarn! ... See my blog entries on SinglesYarn in our Categories for ideas on singles with active twist.