Macadamia nuts have a subtle, buttery rich flavor, with a texture that's more creamy than crisp. Native to Australia, they're grown almost exclusively in Hawaii and are often sold whole but out of their extremely hard shell. Delicious but pricey, they go especially well with tropical flavors, as well as coffee and chocolate.
The closest substitution would be pecans, which have a similar richness and soft texture, but not the same flavor.
You almost never find macadamia nuts still in their hard shells. Fresh nuts should be a pale, yellow-white. Look for plump, unbroken nutmeats and buy from a place that has good turnover.
As with all nuts, taste before using to be sure it's still fresh. If you can only find salted macadamia nuts, you can use them in place of fresh, but give them a quick rinse first. A light roasting in a moderate oven until fragrant and lightly golden will accentuate the flavor of macadamias.
Because of their high oil content, macadamia nuts are very perishable and should be refrigerated in an airtight container.