One of the big challenges of client communication is working with people whose area of expertise differs from yours. The people you work with know their own business (hopefully), but in many cases need to be educated on how to communicate what they need. With projects in the arts, such as graphic design, this presents a level of subjectivity, when trying to communicate a mood or feel, but still it's important.
One thing I've found helpful is books like Graphic Style: From Victorian to Digital, which present artistic styles by historical movement. While these books are not a complete index of every style in existence, if I ask someone to construct something for me, this allows me to pin down exactly what I'm looking for.
These book authors can't keep up with many of the styles available in the digital age, such as skeuomorphism, flat design, or the visual style of infographics, but I do find the popularity aspect of Pinterest helpful for discovering visual styles which are in vogue.
If, however, you search for old images, this will surely disappoint. Enter WikiPaintings - an awesome site which is categorizing these old works in the same fashion as the print books my wife had for art school textbooks. While this site is only a couple years old, it has amassed a sizable collection of images - very valuable if you need inspiration or a tool to communicate with clients.