There are many existing studies of user behavior in simple tasks (e.g., navigational and informational search) within a short duration of 1--2 queries. However, we know relatively little about user behavior, especially browsing and clicking behavior, for longer search session solving complex search tasks. In this paper, we characterize and compare user behavior in relatively long search sessions (10 minutes; about 5 queries) for search tasks of four different types. The tasks differ in two dimensions: (1) the user is locating facts or is pursuing intellectual understanding of a topic; (2) the user has a specific task goal or has an ill-defined and undeveloped goal. We analyze how search behavior as well as browsing and clicking patterns change during a search session in these different tasks. Our results indicate that user behavior in the four types of tasks differ in various aspects, including search activeness, browsing style, clicking strategy, and query reformulation. As a search session progresses, we note that users shift their interests to focus less on the top results but more on results ranked at lower positions in browsing. We also found that results eventually become less and less attractive for the users. The reasons vary and include downgraded search performance of query, decreased novelty of search results, and decaying persistence of users in browsing. Our study highlights the lack of long session support in existing search engines and suggests different strategies of supporting longer sessions according to different task types.