Analog is the process of taking a stream (an Audio or Video for example) and translating it into electronic pulses. Digital on the other hand is breaking the signal into a binary format where the stream is represented by a series of "1"s and "0"s.連続的に変化する量をアナログ量、離散的な量をディジタル量と言います。

Analog computers and other analog systems were in use long before digital devices were perfected. Why then have digital systems supplanted analog systems in most application areas? There are several reasons(Analog vs Digital):

• In general digital techniques offer more flexibility than do analog techniques in that they can be more easily programmed to perform any desired algorithm.
• Digital circuits provide for more powerful processing capabilities in terms of speed.
• Numeric information can be represented digitally with greater precision and range that it can with analog signals.
• Information storage and retrieval functions are much easier to implement in digital forms than in analog.
• Digital techniques allow the use of built-in error detection and correction mechanisms.
• Digital systems lend themselves to miniaturization more than do analog.

A/D変換(Analog-Digital Conversion): 標本化(sampling)->量子化(digitization)->符号化(coding)

Digital logic circuits or switching circuits as they are often called are composed of serial and parallel combinations of switching elements called gates or are implemented via programmable logic arrays or similar devices. From a mathematical standpoint gates are simply open or closed signal paths. From a technological standpoint, they are high-speed electronic switching devices capable of turning on or off in a few nanoseconds.
The positive logic convention uses a high voltage level (H) to represent logic 1 and a low voltage (L) to represent logic 0. The negative logic convention uses L to represent logic 1 and H to represent logic 0. In a mixed-logic system, positive logic is used for some for signals and negative logic for others.
A signal that is set to logic 1 is said to be asserted or active or true. An active-high signal is asserted when it is high (positive logic), where as an active-low signal is asserted when it is low (negative logic). If not asserted, signal is said to be deasserted or negated or false.