Line breaking
The first thing LaTeX does when processing ordinary text is to translate your input file into a sequence of glyphs and spaces. To produce a printed document, this sequence must be broken into lines (and these lines must be broken into pages).
LaTeX usually does the line (and page) breaking in the text body for you but in some environments you manually force line breaks.

\\
Synopsis:

\\[morespace]

or

\\*[morespace]

Start a new line. The optional argument morespace specifies extra vertical space to be insert before the next line. This can be a negative length. The text before the break is set at its normal length, that is, it is not stretched to fill out the line width.
Explicit line breaks in the text body are unusual in LaTeX. In particular, to start a new paragraph instead leave a blank line. This command is mostly used outside of the main flow of text such as in a tabular or array environment.
Under ordinary circumstances (e.g., outside of a p{..} column in a tabular environment) the \newline command is a synonym for \\ (see \newline).
In addition to starting a new line, the starred form \\* tells LaTeX not to start a new page between the two lines, by issuing a \nobreak.

\title{My story: \\[0.25in]
       a tale of woe}

\obeycr & \restorecr
The \obeycr command makes a return in the input file (‘^^M’, internally) the same as \\ (followed by \relax). So each new line in the input will also be a new line in the output.
\restorecr restores normal line-breaking behavior.

\newline
In ordinary text this is equivalent to double-backslash (see \\); it breaks a line, with no stretching of the text before it.
Inside a tabular or array environment, in a column with a specifier producing a paragraph box, like typically p{..}, \newline will insert a line break inside of the column, that is, it does not break the entire row. To break the entire row use \\ or its equivalent \tabularnewline.
This will print ‘Name:’ and ‘Address:’ as two lines in a single cell of the table.

\begin{tabular}{p{1in}{\hspace{2in}}p{1in}}
  Name: \newline Address: &Date: \\ \hline
\end{tabular}

The ‘Date:’ will be baseline-aligned with ‘Name:’.

\- (discretionary hyphen)
The \- command tells LaTeX that it may hyphenate the word at that point. LaTeX is pretty good at hyphenating, and usually finds most of the correct hyphenation points, while almost never using an incorrect one. The \- command is used for the exceptional cases.
When you insert \- commands in a word, the word will only be hyphenated at those points and not at any of the hyphenation points that LaTeX might otherwise have chosen.

\fussy
The declaration \fussy (which is the default) makes TeX picky about line breaking. This usually avoids too much space between words, at the cost of an occasional overfull box.
This command cancels the effect of a previous \sloppy command (see \sloppy.

\sloppy
The declaration \sloppy makes TeX less fussy about line breaking. This will avoid overfull boxes, at the cost of loose interword spacing.
Lasts until a \fussy command is issued (see \fussy).

\hyphenation
Synopsis:

\hyphenation{word-one word-two}

The \hyphenation command declares allowed hyphenation points with a - character in the given words. The words are separated by spaces. TeX will only hyphenate if the word matches exactly, no inflections are tried. Multiple \hyphenation commands accumulate. Some examples (the default TeX hyphenation patterns misses the hyphenations in these words):

\hyphenation{ap-pen-dix col-umns data-base data-bases}

\linebreak & \nolinebreak
Synopses:

\linebreak[priority]
\nolinebreak[priority]

By default, the \linebreak (\nolinebreak) command forces (prevents) a line break at the current position. For \linebreak, the spaces in the line are stretched out so that it extends to the right margin as usual.
With the optional argument priority, you can convert the command from a demand to a request. The priority must be a number from 0 to 4. The higher the number, the more insistent the request.