NAND and NOR gates are universal gates because any other logic gate can be implemented using only NAND or NOR gates. The document provides examples of how to construct NOT, AND, OR, XOR, and XNOR gates using only NAND gates. Similarly, it demonstrates how to construct these common logic gates using only NOR gates. Both NAND and NOR gates are universal because Boolean logic can be represented entirely with either of these gate types alone.