When sawing and drilling, you create burrs and sharp edges on your workpiece that need to be removed; rasps and files take care of these. Using a rasp or a file is often the first step in finishing a project, or prepping a material for the next step.
Step 1: Identifying Files and Rasps
Flat rasp - has large, coarse teeth to quickly remove material
Flat file without a handle - allows you to remove burrs and rough edges
Multi-use rasp and file - has two files and two rasps on one tool
Fine flat file without handle - this tool is for finishing and scratch and gouge removal
Triangle file - allows you to get into corners and clean up edges that have notches
Small round file - cleans small round holes and inside curves
Medium round file - as above, but with coarser teeth
Large round file - as above, but coarser still
For more information:
Wikipedia Rasp Entry
Wikipedia File Entry
Briefly, a file is for detail work while a rasp removes larger quantities of material.
Step 2: Using a File or Rasp
Here, Lynne is filing out the corner of a sign she cut on the plasma cutter. Since she's filing out both sides of the corner, she can butt the file up to both edges. In this situation, you could also use a triangle file to smooth the corner. (She's trying to smooth out a pit from the plasma cutter's piercing move... but that's a different instructable.)