Carbon nanotubes are basically divided into two classes: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Both have very different physical properties due to their structural differences.
The most important feature of SWCNTs is that the wall of the nanotubes consists of a single layer of graphene. In other words, single-walled carbon nanotubes can be described as sheets of graphene rolled into hollow cylinders.
A multi-walled carbon nanotube can be viewed as a concentric array of SWCNTs, i.e. multiple layers of graphene rolled into a tubular shape.