What is the generally accepted difference between SEO and SEM?

SEO deals with all the methods and strategies you use to get higher placement in the SERP.
Search Engine Marketing deals with the tactics associated with your business that drives traffic to search engines, including SEO and PPC.
The term “search engine marketing” is often used by marketers only for paid activities.

SEO focuses on organic/natural search rankings, SEM encompasses all aspects of search marketing.