How to drive a video signal (of any kind) to a CRT monitor? What's the protocol, interface, standard, convention, etc.?
There are two common ways to transmit signals to a video monitor, either analog (as embodied in the VGA interface) or digital (as embodied in the DVI and HDMI interfaces).

If you want to put images on a monitor, by far the easiest way to do it is to wire a VGA or HDMI driver chip to your circuit (the intefaces to both kinds of chips are very similar) and design your FPGA, GPU, DSP or whatever to drive that interface, which typically comprises parallel RGB data (8 to 10 bits per color), along with a pixel clock and HSYNC/VSYNC signals.

What are HSYNC and VSYNC? OK, back to the basics, then. A video display is a series of still images or "frames", typically displayed at a rate of 30 to 120 frames per second. Each frame contains hundreds of thousands to millions of individual picture elements or "pixels". Each pixel is a spot of color, specified as three values for red, green and blue (the "additive primary" colors), and these values need to be transmitted to the display at anywhere from 10 M (million) to 125 M pixels/second. In an analog system, the values are transmitted as variable voltages; in a digital system, they're transmitted as digital words, using anywhere from 4 to 10 bits per color.

By convention, the pixels are transmitted starting at the top left corner of each frame, then "scanning" across the top row. Successive rows follow, from top to bottom of the frame. The VSYNC signal indicates the start of a new frame, while the HSYNC signal indicates the start of each row within the frame.