What are Tier-1 and Tier-2 programming languages?

Question

I was reading this redmonk programming language ranking article. There is mentioned about Tier-1 and Tier-2 languages.

So, I know what are Tier-1 languages.

Also, in one of the similar article, it was said that

Scala may be separating itself from the other Tier 2 languages

My question is,

How are programming languages categorized as "Tier 1" or "Tier 2" languages?

What are the differences between these different tiers of programming languages?

Answer 1

Disclosure: I produced the charts referenced, and all of the mentioned caveats apply.

As other answers have noted, the Tiers are essentially language groupings - they have no formal or technical definition otherwise. When Drew Conway and John Myles White first performed the analysis back in 2010, they noticed that plots of the correlated and ranked languages produced visible clusters of languages. The first two tend to be very distinct, the tier or tiers after much less so.

The tiers are mentioned in the above piece simply because they appear to roughly correspond to observed language usage, and are therefore more reliable than the actual numerical ranking, which must be taken with a grain of salt.

If you want to identify the tiers, then, simply view the full size chart and note the placement of languages within the clustered groupings.