Shaders are little programs which run on the GPU which simply transform inputs into outputs. They are written in GLSL. They dont communicate with one another either. The structure of a shader is shown below:
#version version_number
in type in_name;
out type out_name;
uniform type uniform_name;
void main() {
out_name = stuff;
}
There are number of types including all the usual ones (int, double) but also vectors like vec2 and vec3 to determine 2D and 3D positions. Specific shaders need specific inputs and outputs - for example the vertex shader must always recieve some sort of input along with specifying where the vertex data is located (layout (location = 0)). The fragment shader requires a vec4 colour output since it must generate a final colour. Linking variables together is possible as long as the same name is used.
These are another way to pass data and these are global and can be modified from c++ code as well.
To set it's output, we must assign position data to gl_Position.
To set it's output, a output value is set like out vec4 FragColour and this determines the colour for a vertex.