These are items you buy, hold onto, and hope they'll be worth more someday because they're rare.
Want some examples? Baseball cards, action figure dolls, coins, stamps, comics, antique toys, furniture, and art. These are only some of the things that people collect, hoping that, in the future, a buyer will pay a high price for them.
Collecting may sound like more fun than other kinds of investing, but there are absolutely no guarantees 1) that the item will grow in value 2) that anyone will want to buy it.
Collecting can be very risky – especially if you pay a high price for things you decide to collect, for example, rare coins. While collectibles can increase in value, they can also decrease. You'll find them at the very top of the Risk/Reward Pyramid.
Once you've invested in collectibles, the only way to get your money back is to find a buyer willing to pay the price. Therefore, you may not be able to get your money back when you want it. What happens if you can't find a buyer who is interested? Or what happens if a buyer won't pay you at least the price you paid for the collectible? You're out of luck.
Like stocks, you're betting your collections will appreciate. But there's no interest or dividends to rely on. Collecting is a long-term investment, so you must be patient.