This is discussed in detail in the TeX FAQ ^[1]. Summarising the information given there:

1. You can set \hyphenpenalty and \exhyphenpenalty to 10000, which will stop hyphenation, but as TeX will still try to hyphenate this is not hugely efficient.

2. As Joel says, you can use \usepackage[none]{hyphenat} to select a 'language' with no hyphenation at all. This works fine for a single language document, but not if you want to use babel ^[2] or polyglossia ^[3] for other language-specific effects.

3. Setting \righthyphenmin and \lefthyphenmin to very large values will prevent hyphenation as it tells TeX that it must have more characters in the word than are going to be available. The suggested value in the FAQ is 62.

4. You can set \hyphenchar\font=-1, which will prevent hyphenation for the current font: this is probably not the best way for an entire document but is how it is done for the tt font shape in LaTeX.

Now, of those (2) is probably the best choice. However, what you did not say is why you want no hyphenation. TeX hyphenates when it cannot find a good line break without it, so you get few hyphens in most cases. The risk with no hyphenation at all is that the output looks bad.

I use this and it works great for me in almost all documents:

\tolerance=1
\emergencystretch=\maxdimen
\hyphenpenalty=10000
\hbadness=10000

\begin{document}... yadda yadda\end{document}

A quick google found

\usepackage[none]{hyphenat}

and more useful info here ^[1].

If one uses babel, there's the hyphsubst package by Heiko Oberdiek:

\documentclass[a4paper]
...
\usepackage[german=nohyphenation,french=nohyphenation]{hyphsubst}
\usepackage[german,french]{babel}

provided the distribution knows about the virtual language nohyphenation that has no patterns (both TeX Live and MiKTeX should know it).

If this is not the case, the following hack is equivalent

\makeatletter\chardef\l@nohyphenation=255 \makeatother
\usepackage[german=nohyphenation,french=nohyphenation]{hyphsubst}

(at least if less than 256 languages are already defined in the format, which is quite likely).

TeX will still possibly break lines at explicit hyphens, though.

To explain it better: if you get an error about

Unknown pattern nohyphenation

then the document should be like

\documentclass[a4paper]

\makeatletter\chardef\l@nohyphenation=255 \makeatother
\usepackage[german=nohyphenation,french=nohyphenation]{hyphsubst}
\usepackage[german,french]{babel}

You can use the command:

\raggedright

or the environment:

\begin{flushleft}
\end{flushleft}

§ Paragraph alignment ^[1]

Based on the answers from another post ^[1], I found these settings to be perfect to prevent hyphenation without being ugly:

\tolerance=9999
\emergencystretch=10pt
\hyphenpenalty=10000
\exhyphenpenalty=100

\tolerance=9999 allows as much whitespace as possible.

\emerencystretch=10pt allows some extra whitespace per line.

\hyphenpenalty=10000 disables hyphens completly.

\exhyphenpenalty=100 allows using hyphens which were already present.

By increasing the width of the spaces between words, it is possible to give LaTeX more room to stretch or squeeze a line of text, thus reducing the frequency with which words have to be divided at line breaks. In many documents, the respacing achieved using the command below will virtually eliminate end-of-line hyphenation, without forcing text into the margin.

    \spaceskip=1.3\fontdimen2\font plus 1.3\fontdimen3\font minus 1.3\fontdimen4\font

In this command, \fontdimen2 is the nominal or ideal distance between words, \fontdimen3 is the allowable extension of the inter-word space and \fontdimen4 is the allowable compression.

It is perhaps worth noting also that hyphenation of a given word can be prevented manually by placing it in an \mbox{}.

By using all the answer here, you might be noticed that space after period feels too wide, where you might be not noticed when there is hyphenation.

This is due to latex use more than single space after period. See the discussion in SE answer here ^[1].

Thus, you can add \frenchspacing to remove this behavior if you use \pretolerance=10000, and disable the french spacing using \nonfrenchspacing if you use hyphenation.

Also, if your text contain hyphenation that you write manually, you can use \tolerance=9000 and \emergencystretch=0pt to allow change line between word using that hypen. Otherwise, you can set \tolerance=1 and \emergencystretch=\maxdimen to prohibit change line between words of your manual hypen.

The MWE (based on this ^[2], and this ^[3]) will be:

\documentclass[12pt]{article} 

\tolerance=9000
\emergencystretch=0pt
\hyphenpenalty=10000
\hbadness=10000
\frenchspacing

\begin{document}

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Etiam eleifend tellus
id ultrices feugiat. Sed a risus vitae nisi placerat posuere. Donec ullamcorper
rhoncus purus, a ornare nunc. In tempus elementum tellus a dictum. Orci varius
natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus.
Phasellus pharetra mollis efficitur. Duis urna nunc, molestie vitae ante in,
pharetra hendrerit arcu. Fusce varius lectus vitae leo facilisis, sed ultricies
velit interdum. Nunc volutpat, neque iaculis tempor scelerisque, enim nunc
posuere sapien, vitae tempor quam justo a odio. Suspendisse porta vel ante et
sagittis. Sed sit amet malesuada ligula, id commodo diam. Donec posuere eros et
orci dignissim tincidunt. Donec imperdiet, metus at lobortis rutrum, nisi felis
pretium magna, quis lacinia magna erat eget quam. Duis eget dolor consequat,
porttitor nunc vel, rutrum tellus. Donec semper finibus justo vel elementum.

\end{document}