The fast development of plasmonics have greatly advanced our understanding to the abundant phenomena related to surface plamon polaritons (SPPs) and improved our ability to manipulate light at the nanometer scale. With tightly confined local field, SPPs can be transmitted in waveguides of subwavelength dimensions. Nanophotonic circuits built with plasmonic elements can be scaled down to dimensions compatible with semiconductor-based nanoelectronic circuits, which provides a potential solution for the next-generation information technology. Different structures have been explored as plasmonic waveguides for potential integration applications. This review is focused on metallic nanowire waveguides and functional components in nanowire networks. We reviewed recent progress in research about plasmon generation, emission direction and polarization, group velocity, loss and propagation length, and the near-field distribution revealed by quantum dot fluorescence imaging. Electrical generation and detection of SPPs moves towards the building of plasmonic circuits, where bulky external light sources and detectors may be omitted. The coupling between metal nanowires and emitters is important for tailoring light-matter interactions, and for various potential applications. In multi-nanowire structures, plasmon signal control and processing are introduced. The working principles of these nanowire-based devices, which are based on the control to the near field distributions, will become the design rule for nanophotonic circuits with higher complexity for optical signal processing. The recent developments in hybrid photonic-plasmonic waveguides and devices are promising for making devices with unprecedented performance.