It may seem an odd thing to want to do (after all, one of TeX's great advertised virtues is the quality of its hyphenation) but it's sometimes necessary. The real problem is, that the quality of TeX's is by default largely dependent on the presence of hyphenation; if you want to abandon hyphenation, something has to give.
TeX (slightly confusingly) offers four possible mechanisms for suppressing hyphenation (there were only two prior to the extensions that arrived with TeX version 3).
First, one can set the hyphenation penalties \hyphenpenalty and
\exhyphenpenalty to an 'infinite' value (that is to say, 10000).
This means that all hyphenations will sufficiently penalise the line
that would contain them, that the hyphenation won't happen. The
disadvantage of this method is that TeX will re-evaluate any
paragraph for which hyphenations might help, which will slow TeX
down.
Second, one can select a language for which no hyphenation patterns
exist. Some distributions create a language nohyphenation,
and the hyphenat package uses this technique for its
\nohyphens command which sets its argument without any
hyphenation.
Third, one can set \left- and/or \righthyphenmin to a
sufficiently large value that no hyphenation could possibly succeed,
since the minimum is larger than the the length of the longest word
TeX is willing to hyphenate (the appropriate value is 62).
Fourth, one can suppress hyphenation for all text using the current font by the command
\hyphenchar\font=-1This isn't a particularly practical way for users to suppress hyphenation - the command has to be issued for every font the document uses - but it's how LaTeX itself suppresses hyphenation in
tt and other fixed-width fonts.
Which of the techniques you should use depends on what you actually want to do. If the text whose hyphenation is to be suppressed runs for less than a paragraph, your only choice is the no-hyphens language: the language value is preserved along with the text (in the same way that the current font is); the values for penalties and hyphen minima active at the end of a paragraph are used when hyphenation is calculated.
Contrariwise, if you are writing a multilanguage document using the babel package, you cannot suppress hyphenation throughout using either the no-hyphens language or the hyphen minima: all those values get changed at a babel language switch: use the penalties instead.
If you simply switch off hyphenation for a good bit of text, the
output will have a jagged edge (with many lines seriously overfull),
and your (La)TeX run will bombard you with warnings about overfull
and underfull lines. To avoid this you have two options. You may use
\sloppy (or its environment version sloppypar), and
have TeX stretch what would otherwise be underfull lines to fill the space
offered, and wrap other lines, while prematurely wrapping overfull
lines and stretching the remainder. Alternatively, you may set the
text
ragged right, and at least get rid of
the overfull lines; this technique is 'traditional' (in the sense that
typists do it) and may be expected to appeal to the specifiers of
eccentric document layouts (such as those for dissertations), but for
once their sense conforms with typographic style. (Or at least, style
constrained in this curious way.)