Vectors are used to transfer genetic material between organisms. There are several major types of vectors including viral vectors, plasmid vectors, and shuttle vectors. Viral vectors are very efficient at transferring viral DNA into host cells. Plasmid vectors are small and circular, making them extremely valuable tools. Shuttle vectors can propagate in two different host species. Common vectors include cloning vectors, which are central to gene cloning, and expression vectors, which allow for transcription and expression of genes. Vectors have important applications such as cancer gene therapy and vaccination.