What are tier isps?

Some articles on tier isps:

Kaznet - Internet Service Providers
... The first-tier ISPs with international Internet connections and their own infrastructure are KazakhTelecom, Nursat, Transtelecom, Kaztranscom, Arna, Astel, and TNS Plus ... There are approximately 100 second-tier ISPs that are purchasing Internet traffic from the first-tier ISPs ... Other leading first-tier ISPs, Nursat and Astel, operate terrestrial and satellite-based infrastructure ...