Significant advances in our understanding of neutrophil biology were made in the past several years. A newly discovered mechanism was discovered, the formation of neutrophils extracellular traps (NETs). The structure of NETs is composed of the DNA strand and neutrophil granule proteins. NETs were found to have an association with tumor progression. This review highlights the latest knowledge about the controversial effect on tumors of NETs. Pro-tumor and anti-tumor effects are described respectively. The probable mechanisms of the anti-tumor effect are related to its direct killing of cancer cells or stimulation of the immune system to fight against the tumor. The pro-tumor effect has a correlation with matrix metalloproteinase 9 (MMP-9), cathepsin G, and neutrophil elastase (NE). Moreover, the structure of the NETs makes it able to catch the circulating tumor cells, which could lead to metastasis. This review summarizes our knowledge about the proven roles of NETs in the progression of cancer with particular focus on the components of the NETs, and considers NETs as a potential target for cancer therapy.