There are a lot more web standards than just XHTML.
The idea of "web standards" in design is to pick a set of rules (XHTML 1.0 transitional, HTML 4.01, CSS 2, etc) and make sure your work is valid by those rules.
508 accessibility and W3C's WCAG initiative are guidelines for other things that (as luck would have it) just happen to be almost all covered when you stick with the XHTML standards with some careful coding.
Start with a good book; Designing with Web Standards. It is by no means the definitive guide to web standards, but I think its a hell of a first place to start.
The way I see it, is simply making sure everybody can reach your information. No matter what browser, platform or device they are using. To reach this goal we need these web standards, so every understands each other ;)