The real answer is that it's the amount of actual gold in the alloy... it's measured in 24th's (rather than 100ths, which would be a straight %)... so in other words, 24k gold is 100% gold content (24/24), whereas 10k gold is 10/24 which is about 42% gold content.
Talk her into white gold is prettier, and she will stand out from all the other females that meet somebody, and check the ring while still shaking hands, lol.