I keep finding a WSD version of bikes I'm interested in buying. What does WSD mean?
In this case, it stands for "Women Specific Design".
With regard to bicycles, in some respects, especially the seat, clothing and shoes, women often have different needs than men. WSD bicycles or accessories address this problem.
Many of the big bicycle manufacturers have bicycles aimed specifically at women, sometimes there's an obvious scheme like Trek's where they tack on a "women's" designation, sometimes it's more subtle (for instance, the "Ruby" is the women's version of Specialized's "Roubaix".)
Most are based around some assumptions about women that aren't necessarily true (or that can be just as true for men, especially smaller men). In other words, there are women that will fit better on a "men's" frame, and there are men that will fit better on a "women's" frame. It's vastly more important that the frame fit the rider than that the manufacturer's concept of gender fit the gender of the rider.
In particular, the general assumptions about women that bicycle manufacturers tend to use when designing the frames are:
The women specific stuff can include things such as:
The exact details vary from bike manufacturer to bike manufacturer and model to model.