A certain amount of web traffic is attributed to web bots on the Internet. Web bot traffic has raised serious concerns among website operators, because they usually consume considerable resources at web servers, resulting in high workloads and longer response time, while not bringing in any profit. Even worse, the content of the pages it crawled might later be used for other fraudulent activities. Thus, it is important to detect web bot traffic and characterize it. In this paper, we first propose an efficient approach to detect web bot traffic in a large e-commerce marketplace and then perform an in-depth analysis on the characteristics of web bot traffic. Specifically, our proposed bot detection approach consists of the following modules: (1) an Expectation Maximization (EM)-based feature selection method to extract the most distinguishable features, (2) a gradient based decision tree to calculate the likelihood of being a bot IP, and (3) a threshold estimation mechanism aiming to recover a reasonable amount of non-bot traffic flow. The detection approach has been applied on Taobao/Tmall platforms, and its detection capability has been demonstrated by identifying a considerable amount of web bot traffic. Based on data samples of traffic originating from web bots and normal users, we conduct a comparative analysis to uncover the behavioral patterns of web bots different from normal users. The analysis results reveal their differences in terms of active time, search queries, item and store preferences, and many other aspects. These findings provide new insights for public websites to further improve web bot traffic detection for protecting valuable web contents.