The glReadPixels COMMAND

void glReadPixels(GLint x, GLint y,     GLsizei w, GLsizei h,     GLenum format,     GLenum type,     GLvoid *image);

reads from framebuffer rectangle whose origin is (x,y) and dimensions (w,h), and stores in array image.

The argument format specifies the pixel data elements that are being read.
-- Possible formats include

GL_COLOR_INDEX GL_RGB GL_RGBA GL_RED, etc GL_ALPHA GL_LUMINANCE GL_DEPTH_COMPONENT

The argument type specifies the number type used in the format.
-- Possible types include

GL_BYTE GL_UNSIGNED_BYTE GL_BITMAP GL_SHORT GL_FLOAT GL_INT