URLs tend to be very long, and contain characters that would
naturally prevent them being hyphenated even if they weren't typically
set in \ttfamily, verbatim. Therefore, without special treatment,
they often produce wildly overfull \hboxes, and their typeset
representation is awful.
There are three packages that help solve this problem:
\path command.
The command defines each potential break character as a
\discretionary, and offers the user the opportunity of
specifying a personal list of potential break characters. Its chief
disadvantage is fragility in LaTeX moving arguments. (The
Eplain macros
define a similar \path command.)
\url command
(among others, including its own \path command). The command
gives each potential break character a maths-mode 'personality', and
then sets the URL itself (in the user's choice of font) in
maths mode. It can produce (LaTeX-style) 'robust' commands
(see use of \protect) for use
within moving arguments. Note that, because the operation is
conducted in maths mode, spaces within the URL argument are
ignored unless special steps are taken.
It is possible to use the url package in Plain TeX, with the assistance of the miniltx package (which was originally developed for using the LaTeX graphics package in Plain TeX). A small patch is also necessary: the required sequence is therefore:
\input miniltx
\expandafter\def\expandafter\+\expandafter{\+}
\input url.sty
The author of this answer prefers the (rather newer) url package (directly or indirectly); both path and url work well with Plain TeX (though of course, the fancy LaTeX facilities of url don't have much place there). (hyperref isn't available in a version for use with Plain TeX.)