Suppose a fighter with the Step Up and Following Step feats is standing next to an ogre with ten-foot reach. On its turn, the ogre takes a 5-foot step away from the fighter. The fighter can now use her Step Up or Following Step feat to move up to the ogre.
It is clear what happens if the fighter uses Step Up. She makes a 5-foot step, so she does not provoke an attack from the ogre. However, she also loses 5 feet of movement on her next turn.
What if the fighter uses Following Step to move ten feet (maybe one square sideways, and then one square towards the ogre)? Does she provoke an attack from the ogre for moving within its reach?
What if she uses Following Step to move five feet directly towards the ogre? Does this provoke an attack, or is it treated like a five-foot step?
It seems very odd to me that using this feat would cause you to suffer attacks of opportunity, but I can't think of a reason why it wouldn't.
Best Answer
Following Step appears, to me, to pretty clearly indicate that it modifies Step Up. It's not an either/or proposition - to use Following Step, you must be using Step Up. So, reading them together, Following Step allows you to take a ten foot "5-foot step" which doesn't provoke AoOs and also doesn't subtract from your next turn's options (it still costs your immediate action). While you could theoretically use Step Up without the benefits of Following Step, since feats are optional unless otherwise stated, there's no reason to: Following Step kicks in as part of using Step Up, which means it applies to the 5-foot step granted by that feat.
The following rules citations were stolen from Hey I Can Chan's answer, bold emphasis mine:
The feat Step Up has as its benefit:
The feat Following Step has as its benefit:
Closing note: I know, a 10-foot "5-foot step" doesn't exactly make sense. A 5-foot step is a defined game term, though, which means it works well as a shortcut - it's a quick way to say you can move 5 ft without provoking AoOs. Weirdness like that happens when one rule modifies another - such as allowing a 5-foot step to reach 10 ft.