OWOP is an infinite multiplayer canvas where a large, and active community of pixel artists hang out and draw together. If you're more into scripting there's also a large community of programmers making cool bots and scripts for the game, some notable examples of this are a fully working in-game emulator of pokemon, a fully working in-game calculator and many more.
You paint with left and right mouse button, middle click to pan or use the pan tool to move around the infinite canvas. Press ctrl + middle mouse button or use the pipette tool to copy the color under your cursor. There are also many other tools like the fill bucket and the line tool.
You can also press enter to open the chat, g to toggle the grid, and f1 to toggle the gui.