Level 0 (Subscription): These games require you not only to pay for the game itself, but to pay a monthly fee to be able to play. Example: World of Warcraft.
Level 1 (Retail): These games are paid, but you only need to pay once (not counting DLCs, of course). Examples: Most games, especially console ones.
Level 2 (Premium): These games are “free-to-play”, but very limited if you don’t have a premium account. Playing for free is like playing a demo version of the game. Example: Runescape.
Level 3 (Pay-to-win): The most hated games by the gaming community. Basically, the more you pay, the more in-game advantages you have. Examples: Most free mobile games and facebook games.
Level 4 (Semi free-to-play): You can get anything in-game without paying real money, except for stuff that doesn’t affect gameplay, such as skins, clothes and hats. Examples: TF2, Dota 2, LoL to a certain extent.
Level 5 (True free-to-play): It is impossible to pay IRL money on these games. The developers usually gain money through donations, or ads. Examples: Most kongregate games.