The first number denotes if it's a one day race (1.x) or stage race (2.x).
The second number (or letter code) denotes what level the race is
- GT = Grand Tour
- PT = Pro-Tour
- HIS = Historic
- .HC = Hors Categorie
- .1 = level 1
- .2 = level 2
In theory, that's from important to unimportant. But only in theory. The GT, PT and HIS races count towards the UCI world rankings, so those will get you UCI points. There are a few races that the UCI deems to be worthy of PT status (Tour Down Under, Tour of Poland, Eneco....) that really aren't that prestigious, hard or important.
Other races (those organised by ASO, mostly) are not PT because UCI and ASO bicker like five year olds. That's why a race like Paris-Roubaix is a .HIS race (it has no desire to be PT), whilst it's much more important than most .PT one day races (Ronde van Vlaanderen is at the same level as Roubaix).
So the ranking system is a guideline, but it takes a bit of knowledge (of history and prestige etc) to really know how to order all the races (and any order is always subjective, of course).