Space characters separate words. Two space characters in a row are equivalent to a single space character. The end of line is also equivalent to a space character. (However, a percent sign, %, can end a line without generating a space character.) Generally, space characters at the beginning of a line are ignored.
An empty or blank line between two lines of text defines a new paragraph. Two blank lines are equivalent to one.
$ & % # _ { } ~ ^ \
If entered directly in the input they cause LaTeX to do something special.
You can get the following seven of them into your document by simply
preceding them with a backslash (\):
$ & % # _ { }
To get the other three ( ~ ^ \ ) in
your output requires more work.
Commands may have mandatory arguments, which are input in braces ({}). They may also have optional arguments, which are input in square brackets ([]).
For commands which produce text a trailing blank may be interpreted as the end of the command, which does not automatically create a space, so you may have to do something to create an interword space after this text.
The beginning of the document text follows a \begin{document} command.
Input between the \documentstyle and \begin{document} is called the preamble and typically contains new command definitions, information for the title, declarations which affect things like Typefaces, etc.
The document text (and generally also the LaTeX input) is terminated by a \end{document} command.
It is possible to obtain some of the input from external files, i.e., those which are not the main file input to the LaTeX program.