Primarily Google is a hybrid of two search models plus a hybrid concept.
The original keyword match algorithms.
The random traffic statistical page rank model.
Contextual linking analysis.
Up weighting (strong click through rates, clean / semantic page code ...)
Down weighting (page bloat, slow page load, dead links, disappearing content ...)
Penalties (blatant attempts to manipulate search engines, link farms, hidden content, spamming ...)