Pistachios are the seeds of the pistachio tree. Theyâre usually green and slightly sweet. Theyâre called nuts, but botanically pistachios are seeds. People have been eating them for thousands of years.The kernels can have different colors, ranging from yellow to shades of green. Theyâre usually about an inch long and half an inch in diameter. But if you want to taste one, youâll have to crack open its hard shell first.