Gold Bonded Vs Solid

WHAT IS GOLD FILLED/BONDED?
Gold filled is a solid layer of gold mechanically bonded to a base. US standards require the gold layer to consist of 1/20th (or 5%) of the jewelry’s total weight. The gold is usually 14k gold, but sometimes may be 12k gold (sometimes stamped with 14/20 or 12/20, respectively).
Gold filled has some limitations because of the way it is manufactured; it is only available in wire or sheet form, so it cannot be casted or poured into a mold. Gold filled charms are either flat (made from sheets), or hollow inside. We use gold vermeil whenever we can't use gold filled.
Gold filled jewelry has almost 100x more gold than gold plated jewelry. Since the gold is mechanically bonded to the base rather than plated, it can’t rub off easily. It has the same qualities as solid gold jewelry at a fraction of the cost. Gold filled jewelry is also great for people who are allergic to metals, as the material will not cause an allergic reaction.
US gold filled jewelry is our material of choice; we use gold filled where ever we can, as it’s higher quality than vermeil and gold plated, but still not as expensive as solid gold.
  
WHAT IS GOLD PLATED?

Gold plated jewelry is a very light layer of gold - 0.05% actual gold or less - on top of a base metal (usually brass or copper). The thin layer of gold is plated onto the base metal to create gold plated jewelry.

Since the gold plating is very thin, the gold can rub off easily. It is also more prone to tarnishing when exposed to any kinds of liquids or chemicals. Although gold itself does not corrode, copper and brass will, especially when exposed to liquids (in air, sweat, lotion, perfume, etc). Gold plated jewelry is much cheaper than the other types of gold jewelry because it uses a tiny amount of gold. Gold plated is a good option for jewelry you need for just an occasion or trying out new trends. 
  
WHAT IS SOLID GOLD?
Solid gold jewelry is just as the name suggests. There are many different karats of gold, which determines the amount of gold vs alloy. Pure gold (24k) is too soft and malleable to wear in jewelry, so the gold must be alloyed with other metals to give it strength and durability. 14k solid gold means 14 parts gold (58.3%) and 10 parts alloys (41.7%), while 18k solid gold means 18 parts gold (75%) and 6 parts alloys (25%). The lower the karat, the less expensive it is because it is using less gold.