Mellow,
30 is a perfect score. Average scores are 17-19. A good score would be 20-22. 23+ are generally 99th+ percentile.
The scores that you will see people discussing around here are two: academic average and PAT. The AA is the average of the Bio, Chem, O-Chem, Reading, and Math scores (each scored up to 30).
So, when you see scores such as 21/18, it refers to a 21 average of the academic scores, and an 18 on the Perceptual Ability portion of the exam.
Hope this helps somewhat!