system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a graphics card work?

I've been looking into computer parts as of late and am wondering how exactly a graphics card works, what it does, and what makes one graphics card better than another. I'm practically a 5 year old when it comes to computers, so... ELI5.
gpt
A computer is like a guy doing a lot of math and calculation for you.  We'll call him Brian because he's like the brain of your computer (get it? haha).  The graphic card is like an artist that works with Brian.  We'll call her Gracie.  Gracie can draw whatever Brian asks her to and show it to you.

At the basic level, Brian can simply tell Gracie what color should each spot be on the picture.  He does all the calculations and she just merely executes the change.  Her only job is to remember what color goes where for the whole picture, until Brian tells her that some spots are now in a different color.

But that's a lot of work for Brian and he's got other calculations to do, while Gracie's only job is to paint the picture.  So we came up with better graphic cards that can do more than just coloring spots.  That is, Gracie learned new tricks.  Now Brian can ask her to draw a line between two points and she will, on her own, color all the spots between these points so that it looks like a line.  Or Brian can ask for a triangle and only give Gracie the three corners of the triangle.  She will fill the triangle on her own.

That's better but we can still alleviate Brian's job.  One way to do it is Windows acceleration (or another Operating System).  What happens is that your OS does a lot of the same things when using windows: there's a background image, then there are windows, with a border drawn in a specific way, maybe a gradient on the title bar, specific scrollbars, buttons to close or resize, etc.  Rather than have Brian tell Gracie the full instructions on how to draw each of these all the time, Gracie can learn how to draw those windows.  Now Brian only needs to tell Gracie where a window is, what size it is, what buttons it has, and Gracie will draw the title bar, the buttons, the scrollbars, etc on her own.  Additionally, Gracie remembers what's behind each window, so that when you move a window, she can draw what's behind it without Brian having to tell her.

That's for the operating system.  When it comes to games, Gracie will learn to draw in 3D.  Brian will only tell her where the objects are, what their sides are like in color and texture, where the light is coming from... And Gracie will calculate on her own what parts of the objects are visible or hidden, what is their color according to lighting, how the texture looks like in these specific conditions, if they are transparent or reflective, etc.  Now she's really an artist.

And that's where the difference comes from.  There are better and worse artists.  The better artists will have more tricks up their sleeves: they can calculate reflections, special lighting effects, apply special filters...

A better artist is also a faster artist.  All these calculations require time, and you need many pictures per second for a smooth animation.  Maybe Gracie can only draw five shiny shapes in such a time.  If Brian asks for more shapes, she won't be able to keep up.  Or you can ask her to remove the shiny effect so that she can draw faster.  Or you can hire a new artist that can draw 20 shiny shapes at the required speed.

That's how it is with graphic cards.  Better graphic cards can draw faster, and they have more tricks up their sleeves, which can alleviate the CPU calculations and/or make the image look better.