How to Use a Ruler
M-Short 
The Back
On to the back side of the ruler.
Imperial
Starting on the left again, we see this side is labeled "Inches", you may also notice that the markings for the ruler are not uniform. The first 2 inches are divided into 1/2" and then 1/4", 1/8", 1/16", and 1/32" since sometimes these are important. The other 10 inches are divided by into 0.5", then 0.1" and finally 0.05".
PCB Trace Widths
Under the inch designation you'll also see the definition of mil (1/1000 inch) and then more line width markings ranging from 0.005 to 0.1 inches (5mil to 100mil).
Morse Code
Next, you will see a Morse code cheat sheet. How often is it useful? Who knows, but next time someone starts banging out Morse code on your door in the middle of the night, you'll be thankful you have your ruler.
Digits of Pi
Near the bottom, you'll see π expanded out to more digits than you will need for any project and more than you will need to impress your family and friends with your knowledge of π (actually forget about memorizing π and learn to bake a pie, they'll be much more appreciative). If you are wondering how to type out the π symbol check out the front of the ruler.
Transistor and Diode Symbols
Next, are some transistor diagrams. It seems no one I talked to ever seemed to remember the configuration for transistors, so they are here as well.
Just to the right of them, above the protractor, you'll find drawings of a few diodes as well.
Wire Gauge
Along the bottom, you'll find some wire gauges as well as their corresponding size (in mm). I did check these out on a few and the wire will fit in pretty snuggly in the correct hole. There is also a 1mm hole in the middle even though it doesn't correspond to any wire size.
Roman Numerals
Next is Roman Numerals. Why are they there? Honestly, I don't know, I figured I needed to put something there and it seemed reasonable. To be honest, the only time I've seen Roman Numerals used in the past 5 years, is for the Superbowl. But, if you ever find yourself back in ancient Rome and have a hard time reading the numbers, your ruler can help (although reading the numbers will probably be the least of your worries).
Fraction and Decimal Conversions
On the other hand, the Romans didn't have to deal with changing fractions to decimals. While you all probably know what 1/2 is, who remembers what 1/8 is or worse 5/16. Sometimes you need these numbers, so we've broken out a list of some of the more common value (and added a * for those that are repeating)
Machine Screw and Drill Sizes
Lastly, is the most confusing part of the ruler. Screw and drill sizes. This is mostly confusing, because of the complexity of the subject, you will find plenty of Tap and Drill Charts online, but this should be a decent summary of some of the more common sizes. With a given screw, the drill size you need will vary based on the material you are drilling into as well as the fit you want. The markings on the ruler give the major diameter of the screw (not the diameter of the hole), the screw name, and the drill bit required for 75% thread which is the size of the hole.
That's about it, if you look you'll also find a few logos and a version number and such, but that is the basics of the SparkFun ruler. Get out draw some straight lines, measure things, and convert lots of things.