Carbon nanotubes: consist many carbon atoms arranged in a tube. The carbon atoms are arrayed in a hexagonal lattice similar to the atomic sheets (graphene) in the carbon allotrope graphite. Nanotubes are capped by a hemispherical array of carbon atoms (as seen in Buckminsterfullerene). Nanotubes can form nested structures with greater diameter tubes containing lesser diameter tubes. Nanotubes have great strength and can be excellent electrical conductors or semi-conductors.
The image here is of an un-capped single walled nanotube (SWNT). The carbon atoms are shown as pearlescent spheres drawn at their covalent radius (i.e. just touching)..