New and different trends in networks have been promoted in recent years, among these, Network Functions Virtualization (NFV) has emerged as an initiative from the industry (manufacturers and network operators) in order to increase the flexibility of deployment and integration of new network services within operator's networks. This is achieved through the implementation of network functions such as firewalls, encryption, filtering, load balancing, among others, via software modules. The idea is to avoid using devices based in specific hardware appliances and increase return on investment. Transforming network functions in software allows to run them on different general purpose equipment that could be located in a variety of telecommunications infrastructure, including data centers, network nodes, and even in end-user facilities, also such network functions can be moved or migrated from one equipment to another easily without the need to install new specialized hardware. This article presents a state of the art on network functions virtualization, also it proposes a classification of the main projects that are working under this paradigm. Finally, use cases and future challenges in this area are discussed.