Answer by Will Robertson for Difference between \textwidth, \linewidth and \hsize

I think the simplest way to describe the difference is as follows:

• \hsize is a TeX primitive that should not be usually used in LaTeX
• \textwidth is the (constant) width of the total text block
• \columnwidth is the (constant) width of a single column of text
  (which is the same as \textwidth for a single column document)
• \linewidth is a variable that represents the current size of the line of text, whether inside a column or a minipage or a list

In general, then, it's best to always use \linewidth if you are specifying the relative size of an image or a box, since it will adapt to the current situation.

Note: \linewidth also appears to work in table columns, not just text columns. See this answer for an example where a fixed-width parbox is used within a table cell (actually a multirow cell).