The increasing need for labeled data has brought the booming growth of crowdsourcing in a wide range of high-impact real-world applications, such as collaborative knowledge (e.g., data annotations, language translations), collective creativity (e.g., analogy mining, crowdfunding), and reverse Turing test (e.g., CAPTCHA-like systems), etc. In the context of supervised learning, crowdsourcing refers to the annotation procedure where the data items are outsourced and processed by a group of mostly unskilled online workers. Thus, the researchers or the organizations are able to collect large amount of information via the feedback of the crowd in a short time with a low cost. Despite the wide adoption of crowdsourcing, several of its fundamental problems remain unsolved especially at the information and cognitive levels with respect to incentive design, information aggregation, and heterogeneous learning. This tutorial aims to: (1) provide a comprehensive review of recent advances in exploring the power of crowdsourcing from the perspective of optimizing the wisdom of the crowd; and (2) identify the open challenges and provide insights to the future trends in the context of human-in-the-loop learning. We believe this is an emerging and potentially high-impact topic in computational data science, which will attract both researchers and practitioners from academia and industry.