Note: This thread will be broken down with one post for each group of connections.
There are basically four types of video connections presently used on a computer.
VGA or 15 pin D-SUB is the most common video connection on any computer, be it in your vehicle, on your desk or wherever. It outputs a VGA signal at the resolution specified by the video card.
Composite Signal uses an RCA-style jack. This outputs video at a resolution to be displayed on a television or similar device. Since televisions aren't designed to display as high a resolution as a PC monitor, the display through this type of connection needs to be at the lowest possible resolution for optimal clarity.
S-Video is a higher-quality video output and can display marginally clearer images than RCA composite connections. Again, since the signal is designed for a television, lower resolutions are required for the clearest image.
DVI stands for Digital Video Interface. It's used mostly on higher-end video cards and Macs and is for outputting a digital video stream (all the other types listed are analog) to an LCD panel. The most commonly used connector is DVI-D, shown in the diagram below.
How does this relate to my vehicle computer project?
A lot of people like the idea of using a relatively inexpensive screen that is designed to be used with a DVD player or a video game system for their computer, since they're relatively cheap. Also, many car audio manufacturers make snazzy in-dash motorized screens for the same purpose. These units will only have the RCA Composite or possibly an SVideo connection.
Sure, they can be used as a computer display, but to get an acceptable image, you're going to have to do a lot of fiddling with video settings and even the odds are high that you won't be happy with the result.
Smaller VGA monitors are pretty plentiful and in-dash motorized units that are as slick as the TV screens by the car audio manufacturers are becoming more and more commonplace (finally!). This allows the best visual display, as well as a means of mounting your screen without having to modify your dash or console.
Can I convert one signal type to another?
DVI to VGA adapters are usually included with video cards that have a DVI output. They are readily available from multiple vendors, as well.
Some video cards that have TV-out capability, but no RCA or SVideo conection can use a VGA to RCA cable to convert the signal. There are also scan-line convertors for this purpose in the event your video card doesn't have a TV-out capability.
As far as I know, there is no method of taking an RCA signal and inputting it to a VGA connection. Even so, your PC likely has a VGA output, so this conversion is really somewhat pointless.