Answer by Phil Hirschhorn for Difference between \textwidth, \linewidth and \hsize

One difference is that in list and trivlist environments (i.e., including quote, quotation, and other such environments), \linewidth is smaller than \textwidth by the sum of \leftmargin and \rightmargin. I think I also remember that \hsize is what's used by the routine that breaks paragraphs into lines; since \leftmargin in a list is created by using the \parshape command, that routine doesn't need to be explicitly told about \leftmargin.