Online targeted ads are those shown only to certain users based on interests, demographics, or behaviors. Because targeted ads raise many privacy concerns, many platforms provide ad transparency systems (ATSs) to inform users about this practice. To better understand what current ATSs are communicating to users—and how—we first taxonomized the design and content of 22 of the most popular English-language websites' ATSs as presented to users in the United States. We found substantial differences across ATSs in both the prevalence of transparency-enhancing features (e.g., whether they show users what has been inferred about them) and the presentation of information (e.g., the terminology used, where settings are located). Across all platforms, however, we observed consistent ambiguity about what data is used to target ads and the actual impact of altering settings. To gauge how these different design choices impact users, we conducted an online user study in which 198 participants used their own account to explore the ATS of one of eight representative platforms. We found that many of the questions participants hoped the ATS would answer remained unanswered after exploring the ATS. More broadly, participants found current ATSs simultaneously complex and lacking key details. We pinpoint ATS design decisions that best support users.