RGB (Chroma) encoders create s-video or composite video from RGB. Most post-NES/Famicom consoles have one inside, though many (like the GameCube and PCEngine) use a dedicated encoder that cannot be separated from the machine. The most common encoders are from Sony, the CXA series powered just about every pre-Dreamcast system. The CXA1145 didn't offer real S-video, but the CXA1645 and CXA2075 did (see Micomsoft XMD-3). The last chip, the CXA2075, is the best in the series but is very rare.
Chroma decoders disassemble composite or s-video and rebuilt an RGB signal. All TVs perform this function, and many arcade machines - particularly DVD or LD machines, to drive the RGB monitor from other signals.
Transcoders convert from one video format to another. The most common use for a transcoder (for the users of this site at least) is to create component video from RGB, or vice-versa.
Upscanners usually create VGA signals from TV-rate signals (31kHz+ from 15kHz). You can also downscan, such as creating TV from VGA, but since most modern video cards offer this option it's becoming less common.
Also called VGA boxes, among other things. Micomsoft's X-RGB series are the best available upscan convertors, as they are typically the only ones that will work with RGB signals.