Short reply: IAF is Initial Approach Fix. It marks the start of the Initial segment of the approach. IF is the Intermediate Fix, which marks the start of the next, or Intermediate segment. For completeness, though not in yr question, the start of the Final segment is marked by the FAF or Final Approach Fix.
PANS-OPS refers. It's not so much a matter of what you can do at each stage, as how the obstacle clearance changes as you get closer to the runway.