1 GOLD BAR = 1 GOLD.
You will receive 999 GOLD in GOLD BARS.
You may sell the gold bars in game to any vendor and receive 1 gold per gold bar.
Gold is used to purchase a few items in game (vendors), but most importantly to donate to your guild coffer and when you remove enchantments from armor/weapons/runestones slots.
If you work professions, gold is extremely important for your crafting and gathering.