These variants of mouse-1 select entire words or lines at a time. Emacs activates the region around the selected text, which is also copied to the kill ring.
Double-clicking on a character with symbol syntax (such as underscore, in C mode) selects the symbol surrounding that character. Double-clicking on a character with open- or close-parenthesis syntax selects the parenthetical grouping which that character starts or ends. Double-clicking on a character with string-delimiter syntax (such as a single-quote or double-quote in C) selects the string constant (Emacs uses heuristics to figure out whether that character is the beginning or the end of it).
Double-clicking on the beginning of a parenthetical grouping or
beginning string-delimiter moves point to the end of the region,
scrolling the buffer display forward if necessary to show the new
location of point. Double-clicking on the end of a parenthetical
grouping or end string-delimiter keeps point at the end of the region
by default, so the beginning of the region will not be visible if it
is above the top of the window; setting the user option
mouse-select-region-move-to-beginning to non-
this to move point to the beginning of the region, scrolling the
display backward if necessary.