CGA stands for color graphics adapter, the first IBM video card to permit graphics on the screen. We’re lucky they’ve come out with better models, because CGA graphics are gawdawful crude. With a CGA, your screen can show up to 640 dots across by 200 dots up and down, with only one color. Even at that maximum resolution, pictures look really blocky and out of proportion. Pictures will look even more blocky if you want 4 colors on the screen at once, since you’re then limited to 320 dots across and 200 down. If you can tolerate a totally chunky display of 160 by 200 dots, you can get a maximum of 16 colors on a CGA. Wow!
A CGA can display text too, but the characters are fuzzy looking and squished together, so they’re hard to read. And you may see an annoying sparkling effect called snow when you scroll the text. So don’t buy a computer with a CGA. And if someone gives you one, put in a VGA instead.
Many other companies besides IBM have produced video cards that work just like a CGA. Some pcs, including most laptops made prior to 1990, come with built-in CGA-Compatible circuitry. These variations are generically referred to as CGAs or CGA systems, trademarks notwithstanding. And since people don’t look at the video circuits too often, they generally end up using the term CGA to refer to their monitor, as in “I have a CGA screen.”