This note provides background on Title 47 US Code § 230, the section of federal law that protects internet companies from liability for the content shared by users, and for the moderation and even removal of that content if considered objectionable. So central is this law to the debate over online speech and content policy that it is commonly referred to simply as "Section 230," even though nearly every one of the 54 titles in federal law also contains a section 230. Passed as part of the Telecommunications Act of 1996, Section 230 ended a "moderator's dilemma" that resulted from a 1995 court decision. If that decision had not been overturned, it would have meant internet companies that restricted access to objectionable content became liable for any and all content on their platforms. This technical note is particularly useful when used to facilitate discussion of the case "Trump v. Twitter, et al." (UVA-S-0360). The case explores the period in 2020 in which the social media activity of Donald Trump, then president of the United States, started to be moderated by some social media companies, receiving warning labels and even being removed for violating content guidelines.