Wikipedia defined AA as "...a graphic design technique that uses computers for presentation and consists of pictures pieced together from the 95 printable (from a total of 128) characters defined by the ASCII Standard from 1963 and ASCII compliant character sets with proprietary extended characters (beyond the 128 characters of standard 7-bit ASCII). The term is also loosely used to refer to text based visual art in general. ASCII art can be created with any text editor, and is often used with free-form languages. Most examples of ASCII art require a fixed-width font (non-proportional fonts, as on a traditional typewriter) such as Courier for presentation."
It is never actually known when ASCII Art was originated, but the oldest example of an ASCII art are creations by computer-art pioneer Kenneth Knowlton from around 1966, who was working for Bell Labs at the time.
AA Characters are text generated characters that are known, and generally well received by the Japanese, and also popular in the largest bulletin board system, 2ch - a lot of ASCII arts are created by the denizens of 2ch and has since spread wide across many anime blogs and even chatrooms.
Tags that are tagged "AA" are illustrations that are highly related to these creations, such as Monā.
Monā (or Mona)
Eight Headed Monā
AA based meme that are made famous by 2ch due to them spreading wide across the boards, and eventually, outside 2ch and sometimes even the world.
Note: These translations are not final, tags can be subjected to change at any time.
Don't Look Here (こっちみんな)
Heaven Situation (ヘブン状態)
Yada Yada (ヤダヤダ)
I'll Make Fried Rice (チャーハン作るよ!)
Why Has it Turned This Way (どうしてこうなった)
I Refuse (お断りします)
BRB Going to Konbini (ちょっとコンビニ行ってくる)
Such Reckless… (無茶しやがって…)
Let's Take it Easy (ゆっくりしていってね)
Existing images(Illustrations, pictures, etc) or screenshots that are traced and converted as an AA