How is voltage level used to distinguish between binary digits?

Binary digits are represented in a variety of ways inside a computer. Random access memory (RAM) typically uses a capacitor and a transistor to represent a single bit. To set a bit, the transistor fills the capacitor with an electric charge. To clear a bit, the transistor drains the charge. The transistor can achieve this "switching" extremely quickly.

However, a capacitor cannot hold its charge for long because the transistor "leaks". In order to maintain state, memory must be constantly refreshed at regular intervals. On each refresh, if a capacitor's charge is more than half full, the transistor refills it.

The refresh rate obviously needs to be faster than the leakage rate, however the more time spent refreshing memory (the refresh overhead), the slower that memory will be because the refresh has to be interleaved into the normal memory accesses. In older DRAM, the refresh was as high 10%, but today it is less than 0.5%.