In some ways, gold behaves like real-world currency, with Blizzard's servers acting as the mint that creates and destroys the gold. Gold is created, that is to say it enters the economy, in the following ways:
As rewards:
By completing quests
By completing Dungeon Finder, Raid Finder, and Scenarios
As guild rewards
By selling items to vendors
As loot
Transfers via server and/or faction
The first three are an unlimited resource of gold; as long as players keep repeating these, the server will keep creating gold and giving it to the players. The final source is simply a transfer of one economy to another either via a player or guild transfer, or through sales across the neutral Auction House.
Every expansion sees gold raised from these sources; every time the level cap goes up, you get more gold from quests, mobs drop more loot, and higher item levels sell for more gold to vendors. This increase in gold production causes inflation as the server just produces more and...