Well, I'll try to put it as simply as I can. Basically, it's just putting different weightings on different factors and adding them all up to produce an overall rating.

Have a go at the AAP Neurals. What you do is simply give a weighting of between 0 and 5 to the following factors:
CP - Career performance assessment based on weight/class
CF - Current form measured by class/weight
TIM - Revolutionary time assessment
JA - Jockey ability
TA - Trainer ability
JT - Jockey/trainer combination
WT - Wet track performance
Crs - Course suitability
D - Distance suitability
$ - Prizemoney earned
BP - Barrier position (course & distance)
DLR - days since last run

Once you've put your settings in, it will come up with a rating for the whole field based on those settings. Have a try. You'll find the "Neural Algorithms" on the right side of the page:

http://www.aapracingandsports.com.au/racing/