How to Read a Horse Racing Form (Excellent site)
Horse Racing/How to Wager 101
Horse Racing Dictionary of Terms
Racing Terms National and International
From the Brits: US tracks follow.
You may also see theses abbreviations:
United States (US) tracks:
Picking A Winner by Reading and Understanding the Racing Form
There is so much information packed into a horse racing form that it is dizzying, overwheling and confusing! By learning how to read the race card you will have information necessary about the horse's performace to enable you to make an informed decision when betting. However, there are so many variables in a horse's performace that the information provided is only a helpful overview of past and perhaps future performace. Below is helpful information to help you understand a Horse Racing Form for horses running on the flat on a track, i.e., not over jumps, etc.
"How to Read a Horse Racing Program
What does a horse racing program look like?
The Racing Form or Race Card - Get on your reading glasses or have a magnifying glass near because the font is size 8! There is so much information provided in such a small space.
Image: Keenland Racetrack Infographic
Detailed info graphic created by Keeneland Racetrack
How to Read a Horse Racing Program"
Beginning Information to Understand:
Without getting bogged down in the details you can determine a lot from these simple snippets of information by doing the following things:
The Records...
Past Performances
In general you will be provided with the following details from left to right:
Extra details may include:
Take particular note of the details with italics. ...Although there will be a series of numbers detailing the order of calls, singling out the final number which will designate the horse's position at the finish line should be enough to make a judgement.
The other important facts are the race restrictions, course type and type of race. These are all variables which must match up with the race you are betting on to make your predictions as accurate as possible. The past performance results must be taken with a pinch of salt. Although they will give punters concrete facts to make their prediction often they can be misleading if you skim over the details....
Race Type Code
There are many different types are race codes, here are the three most common ones for top races:
Course Conditions and Course Symbols
...Beyond the horse and the jockey its important to consider what the terrain is like when you are analyzing and horse racing program. The course symbols are easy to overlook because they are designated by a series of very obscure abbreviations which can easily wash over a racegoer's head. If a horse won on a course which does not resemble the one they are racing on at a later date, then all bets will be off. After you have checked whether the horse's prowess is on Turf or Hurdle, it is important to verify the condition of the track. It may have been sloppy, muddy, hard or good, depending on the day. All of these minute details add up to create a complete picture of a successful horse.
Race Restrictions...
This is only really important if a horse has just turned 3 and therefore has just begun to qualify for more challenging races. The four symbols you need to remember are:Medication and equipment symbols...
In practice this information is not required to secure a win. It is just useful to identify these symbols so that you are not thrown when you see these abbreviations on the program. If you are more advanced you may want to see whether a horse has improved automatically when they were put on Lasix or Bute.
Claiming Races - What is a claiming race? A race in which the horses are literally for sale. Any claims must be made before the race, and the new owner assumes possession following the race. 
Claiming Race/What is It?
For More Information:
Picking a winner by reading the form