Wireless sensor networks are different from traditional networks and highly dependent on applications, so traditional routing protocols cannot be applied efficiently to them. Therefore many routing protocols for wireless sensor networks are studied. After describing the characteristics of wireless sensor networks, the classification standards for routing protocols are presented. Then the key mechanisms of the existing representative routing protocols are analyzed and their classifications, characteristics and application areas are compared. Finally, the important features that good routing protocols possess are summarized, and the future research strategies and trends.