The distinction stems from research on the "Focus Theory of Normative Conduct" (e.g., Cialdini et al. 1990)
Injunctive norms refer to the perception of what ought to be, or as you state, perceptions of what is approved or disapproved by others.
Example: If you think that other people would consider tax fraud as something that is morally wrong, you would perceive an injunctive norm.
Descriptive norms refer to the perception of what is. Or, in other words, perceptions about how people do in fact behave.
Example: If you think that most people engage in tax fraud, that would be a descriptive norm.
As you can see, injunctive and descriptive norms are distinct concepts. Sometimes they may overlap (tax fraud is wrong, and nobody does it). However they may also conflict (tax fraud is perceived to be wrong, but everyone does it). Of course they can also be about positive things (it's a good thing to eat 5 portions of veggies each day vs. nobody does that).
Cialdini, R. B., Reno, R. R., & Kallgren, C. A. (1990). A focus theory of normative conduct: Recycling the concept of norms to reduce littering in public places. Journal of Personality and Social Psychology, 58, 1015–1026. doi:10.1037/0022-35184.108.40.2065