We discussed the underlying technology of Flash memory in How ROM Works, but here's a quick review:
Flash memory is a type of EEPROM chip. It has a grid of columns and rows with a cell that has two transistors at each intersection (see image below).

The two transistors are separated from each other by a thin oxide layer. One of the transistors is known as a floating gate, and the other one is the control gate. The floating gate's only link to the row, or wordline, is through the control gate. As long as this link is in place, the cell has a value of 1. To change the value to a 0 requires a curious process called Fowler-Nordheim tunneling. On the next page, we'll talk about tunneling.