Shockwave was Flash's larger brother, feature wise. Shockwave stuff could contain Flash files, but Shockwave could do things Flash couldn't, or could do them earlier. (And if I remember correctly Shockwave came first, and when Macromedia aquired it they then build Flash)

It was two separate products with separate origins. Macromedia had Shockwave and then they purchased Flash (which was called Splash then) and branded it Shockwave Flash.

Shockwave was somewhat similar to Flash, but it was developed for the "multimedia" CD-ROM's, which meant the files was generally too big for online. Flash was much more compact which is why it won out.

Shockwave = plugin for playing Macromedia Director content. Director was a GUI builder for interactive multimedia apps which were programmed in a custom language called Lingo.

Flash was originally called FutureSplash, if I recall correctly. When Macromedia bought it they rebranded it to fit their general branding theme, hence the confusion. Flash wasn't really intended to be an app platform, it started out as a vector animation format, but later they added scripting using a dialect of JavaScript called ActionScript.

Most users ended up with the Flash plugin but not the Shockwave plugin. Macromedia Director was huge in its day - my first programming job involved writing Lingo - but it died out pretty quick when the internet started taking off.