Bascially "you're" is short for "you are", and "you're, your, and yall" are some version of the root word "you". However you forget the last but not least known of the group "YUR". It was invented as an attempt to shorten the amount of time one has their particular mouth open when speaking to others of the same ilk.
An example of commentary between two kissing cousins would be "yur gonna geet it now"...