A very basic one, essentially indicating the type of notebook, based on the thousand number:
1xxx, 2xxx-> Low-end basic models.
6xxx (and before that the 5xxx models)-> Mid-range performer.
9xxx (and before that 8xxx)-> High-end performance machines and gaming notebooks.
7xxm (6xxm, etc)-> 'm' to signify 'mobile', these are the thin and light portable machines.
Then obviously as new models are released the secondary numbers increase, for example the i9400 replaces the i9300, etc.