For the technically inclined

Apple Reveals More Details of Touch ID for iPhone, iPad & beyond (Patently Apple)

Generally, capacitive fingerprint sensors may be used to determine an image of a fingerprint through measuring capacitance through each capacitive sensing element of a capacitive sensor. Thus, fingerprint ridges provide a higher capacitance in an underlying capacitive sensing element than do fingerprint valleys.

Capacitive fingerprint sensors come in at least two varieties, namely active and passive. Active capacitive sensors are often used in electronic devices to provide biometric security and identification of users.

A long discussion, based on Apple patent filings, of what Apple's future fingerprint technology may look like follows.