Shadertoy is a website for demoing / sharing
shaders written in WebGL.
WebGL
is a variant of OpenGL that runs in a web browser. The latest version is 1.0, which is based on
OpenGL ES 2.0, which itself is based on OpenGL 2.0 without deprecated
functionality. So WebGL is kind of like
an older version of OpenGL.
Shadertoy
renders a fullscreen quad, and allows you to program a fragment shader.
Water 2D