What is the difference between a tertiary road and a minor/unclassified road?
thank you! 
In the UK, “unclassified” is a separate class of roads.
In all other countries, it is usually seen as the lowest level of the road network.
Tertiary roads typically connect small villages.
It is probably explained better on our wiki.
You can follow the links on that page to get an even more detailed description of the different types of roads.
There are regional differences too; check your country’s wiki page for guidance.
Some places (such as the “oilpatch” of western Canada where I live) have few to no “small villages”, but plenty of roads to plenty of destinations, mostly oil wells. The more major of these - covering more distance, serving multiple destinations - could be considered tertiary.