Section is root type container element that allows you to group several rows. You can insert section into root canvas only (section element can not be inserted into any other element, including row). Section element can contain only row elements.
Row is the main content element of Visual Composer. Rows are divide your page into logic blocks with columns, columns later will hold your content blocks. Rows can be divided (eg. 1/2 + 1/2, 1/3 + 1/3 + 1/3, etc).
Columns are part of the row and they hold your content elements inside them.