Quite recently, it was found that metal wires can effectively guide terahertz radiation. Based on the fact that the absolute values of the relative permittivities of metals in the spectral region of terahertz radiation are huge, we here analyse the properties of this kind of waveguide and explain the related experimental results. In particular, we show that the observed waveguiding is due to the propagation of an azimuthally polarized surface plasmon along the wire. Some related aspects, such as the choice of metal and the slowly decaying modal field, are also discussed. In particular, we show that, if a copper wire with a radius of 0.45 mm is used, the attenuation coefficient is smaller than 2x10-3 cm-1 in the whole range of 0.1~1 THz.