Minecraft wiki entry for Enchanting, associated text: An enchanting table indicates the item can receive the enchantment legitimately in Survival mode by using an enchanting table.
An anvil indicates the item cannot receive the enchantment legitimately in Survival mode by using an enchanting table (an enchanting table never offers the enchantment for the item), but can receive the enchantment by another method (for example, using an anvil to combine the item with an enchanted book or another enchanted item; note the enchanting table could still apply the enchantment to the item as a random additional enchantment).