I have been looking but can't find anything that explains it comprehensively. Thanks.
This is a great question!
I don't believe there is an authoritative list, rather the classification of tiers will depend on the context. Disclaimer: the below is my opinion.
To try to put it simply, for ad networks, the Tier 1 countries will be those that have substantial volume and high competition/revenues/click prices.
I'd say the tiers are more about advertiser competition and revenue generated than anything else.
Tier 1 countries usually include the major English-speaking countries, France/Germany, and Scandinavian countries. E.g. US, UK, CA, AU, DE, FR, NO, FI, SE.
New Zealand is sometimes T1, sometimes T2 - we are an affluent country but quite small, so often don't have the volume/competition to be considered Tier 1.
Tier 2 often includes large western European countries, Spain, developed Asian countries like Singapore, HK, Japan, etc.
Tier 3 is generally LATAM, China, and South East Asian countries. These tend to have massive volume, but much lower CPMs/CPCs and lower revenues, much of it being due to the lower affluence of the populations.
Here are some lists that may help:
http://www.brusmedia.com/country-tier-targeting/
https://tapfortap.com/tiers
Does that explain it well enough?
We will often tell people to try Tier 3 countries and avoid Tier 1 on X traffic source when starting out. To frame this differently, we are saying: don't attack the highly competitive markets where there are lots of advertisers, high traffic costs, and greater saturation of the market with affiliate offers.
I'd agree with the above!
Tier 3 would also usually include African countries.
I'd say Brazil is a Tier 2 these days, but others might disagree.
Yes I would say Brazil is now Tier 2 as well as probably Argentina...
But this would depend on the traffic source/vertical you're tackling I guess.
@zeno: Thanks a lot. That explains it very well. I'll bookmark those links!
@caurmen: Thanks for the input as well.