Crystal technology used to power any number of advanced technologies, utilized by the Ancients, the Ori, and also the Goa'uld (who cannibalized much of their technology from what the Ancients left behind). It functions like a long-term battery or capacitor, and is likely rechargeable -- though they can become entirely depleted.
Crystals are found inside the D.H.D. that provides power to a Stargate, as well as ships such as the Tel'tak, and unique inventions such as Merlin's computer. The Ori also use them to power their warriors' staff weapons, and likely other technologies.
Line In the Sand - Mitchell and Carter cannibalize an Ori staff weapon crystal to activate Merlin's computer.