When talking about jewelery, what does it mean to be gold filled? gold plated? What's the difference between the two?

ITWP,

gold filled means(as the name suggests!) that a crevice, or some hole in the design, has been filled up by soft-(pure)gold or a mixture of gold.

Whereas, gold-plating means to cover the ornament with a layer of gold
through the process of electroplating.
[You have some knowledge of electrolysis, right? Well, it's the same thing. The jewellry/whatever is placed at the cathode and the metal/element to be 'plated' on top of it is at the anode.]

For a description of the terms used in gold and silver jewelry, click this Gold facts

Now they have a process called, I think, "Permabond", during which a very very VERY thin layer of gold is bonded to stainless steel. You can buy jewelry made this way CHEAP. I bought 4 gold necklaces for $40 but they itched my neck, so I gave them away. Looks just like real gold, which it is, what you see, just like when you look at somebody all you're seeing is dead skin.

Catty (who needs to go to bed now)