Free Cards - These cards are absolutely free.
Highlighted Cards - You may highlight your card for USD $10 to promote it above free cards for six months.
What is the difference? - Free and Highlighted cards are completely identical. The only difference is that Highlighted cards will always appear before Free cards on search results, giving them more exposure. When a Highlighted card expires after six months, it will become a free card again. At this point it can be highlighted again, or remain as free.