Aliens is a text-only clone of Space Invaders written for the CP/M operating system and made to be operated on the early Kaypro line of luggable computers.
Aliens was made within the limitations of Kaypro's early text-only computers. As a result, the aliens are made of letters and punctuation marks. Each level was given a name, displayed on top of the screen, such as Blood Bath and We Come in Peace.
Along with Star Trek, CatChum and Ladder, Aliens was one of the games that came with the software bundle of the early Kaypro computers.
Source: Wikipedia, "Aliens (Kaypro video game)", available under the CC-BY-SA License.