In the era of big data, systems need to process massive data efficiently to meet performance requirements of applications. In-memory computing technology can improve performance of massive data processing significantly by utilizing memory and avoid I/O operations. However, the technology faces a series of challenges that need to be solved. This paper analyzes characteristics of in-memory computing technology, lays out its classification, and introduces principles, related works and hot topics of every category. In addition, several typical applications of in-memory technology are introduced. Finally, challenges and opportunities for in-memory computing are elaborated.