Gold is important in World of Warcraft,you need money to buy skills, items,armor. There are actually two different situations in game,gold farming and gold making.
Gold farming is a simple action,just staying in lower level areas,killing mobs and gaining loot items or materials and then selling them on the auction house for profit. It always takes much time and effort.It's available for lower level players as well. Besides,Make full use of your secondary professions,you can make gold by mining, cooking and skinning as you are leveling up and questing.You can sell food,mining ores and skins on the Auction house.
Gold farming is easier for high level players,there are likely to be many quests left in both Icecrown and The Storm Peaks for level 80 quests.You can earn 500-2000 gold by completing most of those quests,since the experience awarded is replaced by a higher gold amount in compensation.Doing daily quests is main way to make gold for most level 80 players.
Making gold in World of Warcraft is not as difficult as it seems to be,if you spend enough time and make more effort,you can do it as well.