Complex sentence: Nanotubes are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs), that are additional graphene tubes around the core of a SWNT, much like the Russian matroyska dolls.
Split into simpler sentences: Nanotubes are categorized as single-walled nanotubes (SWNTs) or multi-walled nanotubes (MWNTs). The latter have one or more outer tubes successively enveloping a SWNT, much like the Russian matroyska dolls.