Water blocks are most often made out of metal that is exceptional at conducting heat, like copper and aluminum. These solid blocks come in both piped and sink design. The former design involves having a groove or tube cut into the top of the block which connects to the tubing and allows the fluid to simply flow through. The heat sink design more closely resembles a standard heat sink used in air cooling, which attempts to create maximum surface area in a small space in order to transfer the highest possible amount of heat. The sink design is supposed to be more effective, though in actual testing the numbers are so close that it doesn't usually justify an extra cost. Some water blocks appear to be solid, and all you see are the tube fittings that lead the liquid in and out. Others are a bit more flashy, with plexi or glass blocks sealed over the top, creating something of a window. They may also house LEDs for added show.
Water blocks, like their heat sink counterparts, are physically specified to fit on certain CPUs, GPUs, chipsets, etc. When shopping for a water block, whether it is for your processor, video card, memory, or other component, you need to ensure that it is compatible with your hardware. Some water blocks, particularly those for CPUs and GPUs, will often support multiple types, taking advantage of various adapters, fittings, brackets, and other hardware to achieve compatibility. It is important to note, though, that if you opt for a "universal" water block, or one that supports a variety of hardware, that it will usually add several arduous and often frustrating steps to the installation process.
Choosing a water block for a video card (GPU) can be somewhat frustrating as well. There are water blocks out there that are compatible with almost any GPUs that carry very reasonable price tags. On the flip side, there are water blocks that are designed for one specific video card that are 2-3x more expensive. Is it worth it? The fact is that there are more things on a video card that need to be cooled than just the GPU. The memory chips and voltage controllers also put off significant heat, and will shut down the card and/or computer if overheated. Stock fan and heat sink assemblies are designed to cool all components of the video card, and an effective water block should do the same. Keep this in mind when choosing between a universal or proprietary VGA block - unless you have a good alternative for cooling the rest of the card, the one designed specifically for it is likely worth the money.