How To Understand Levis Jeans

Levis 501 Original Jeans - Premier Finish

Levis Original Jeans - Global Blues

I really don't understand the numbering that Levi's has distinguishing there different types of jeans...I was on there website today looking for a pair of skinnys and the 500's and 529's and all that jazz is confusing me!!

I know that 501's are the Originals, but after that I can't tell what makes a 505 different from a 520. Some one please shed some light!