The rapid development of wireless infrastructure and data acquisition technologies contributes to the explosive growth of data, especially trajectory data with rich information. Trajectory data, which records locations of moving objects at certain moments, has long been an important means of studying human behavior and solving traffic problems. In this paper, we mainly introduce the trajectory data from the perspective of applications and services. According to the degree of data structured, we divide the trajectory data into explicit trajectory data and implicit trajectory data, and describe each type in detail. Then, we introduce the applications of trajectory data from travel behavior, travel patterns, and other aspects. Combined with case studies, we provide a description to the services of trajectory data in terms of transportation administration and commercial service. Finally, we focus on challenges in trajectory data, including privacy protection, human mobility causality, and emission reduction.