Aka: AMSCII, AmstradSCII, CPC charset
Amstrad ASCII is the character set used by the Amstrad CPC series of 8-bit computers (see: Wikipedia). It consists of 256 characters. The first 32 characters are control characters which are usually not visible. Up to character 128 the characters are very similar to standard ASCII. All characters above 128 are graphical characters which can be used for simple drawings.
Here the complete charset including control codes:
The Amstrad CPC series has 3 official video modes. They are (wiki):
Standard mode is mode 1 with 320x200 pixels and 4 colors. Each character can individually select one of these colors as foreground and also as background color.
Amstrad ASCII Art is the composition of art works out of Amstrad ASCII characters.
In contrast to Amstrad ASCII Art following art forms are much better known and wider spread: ANSI Art, ASCII Art, PETSCII Art,...
ANSI and ASCII are very popular on the PC.
The Amiga also has his kind of ASCII art.
The C64 is well know for it's PETSCII which is used as art form but also in games, demos, etc.
For the Atari 8-bit line there is only a few releases, that use Atari ASCII like the demo 16384.