People increasingly use videos on the Web as a source for learning. To support this way of learning, researchers and developers are continuously developing tools, proposing guidelines, analyzing data, and conducting experiments. However, it is still not clear what characteristics a video should have to be an effective learning medium. In this paper, we present a comprehensive review of 257 articles on video-based learning for the period from 2016 to 2021. One of the aims of the review is to identify the video characteristics that have been explored by previous work. Based on our analysis, we suggest a taxonomy which organizes the video characteristics and contextual aspects into eight categories: (1) audio features, (2) visual features, (3) textual features, (4) instructor behavior, (5) learners activities, (6) interactive features (quizzes, etc.), (7) production style, and (8) instructional design. Also, we identify four representative research directions: (1) proposals of tools to support video-based learning, (2) studies with controlled experiments, (3) data analysis studies, and (4) proposals of design guidelines for learning videos. We find that the most explored characteristics are textual features followed by visual features, learner activities, and interactive features. Text of transcripts, video frames, and images (figures and illustrations) are most frequently used by tools that support learning through videos. The learner activity is heavily explored through log files in data analysis studies, and interactive features have been frequently scrutinized in controlled experiments. We complement our review by contrasting research findings that investigate the impact of video characteristics on the learning effectiveness, report on tasks and technologies used to develop tools that support learning, and summarize trends of design guidelines to produce learning videos

翻译：人们日益将网络视频作为学习资源。为支持这一学习方式，研究人员和开发者持续开发工具、提出指南、分析数据并开展实验。然而，视频应具备哪些特征才能成为有效的学习媒介，目前仍不明确。本文对2016年至2021年间257篇视频学习相关文献进行了全面综述。本综述旨在识别已有研究探讨的视频特征。基于分析，我们提出一个分类体系，将视频特征及情境要素归为八类：（1）音频特征；（2）视觉特征；（3）文本特征；（4）讲师行为；（5）学习者活动；（6）交互特征（测验等）；（7）制作风格；（8）教学设计。同时，我们归纳出四个代表性研究方向：（1）支持视频学习的工具方案；（2）控制实验研究；（3）数据分析研究；（4）学习视频设计指南的制定。研究发现，最受关注的视频特征是文本特征，其次为视觉特征、学习者活动及交互特征。支持视频学习的工具最常利用文字转录稿、视频帧与图像（图表及插图）。数据分析研究中，学习者活动主要通过日志文件进行深入探讨，而控制实验则频繁考察交互特征。我们通过对比研究结论来完善综述：分析视频特征对学习效果的影响，报告开发学习支持工具所涉及的任务与技术，并总结制作学习视频的设计指南趋势。