To get a straight right margin in the output, LATEX inserts varying amounts of space between the words. It inserts slightly more space at the end of a sentence, as this makes the text more readable. LATEX assumes that sentences end with periods, question marks or exclamation marks. If a period follows an uppercase letter, this is not taken as a sentence ending, since periods after uppercase letters normally occur in abbreviations.
Any exception from these assumptions has to be specified by the
author. A backslash in front of a space generates a space that will
not be enlarged. A tilde `~' character generates a space that cannot be
enlarged and additionally prohibits a line break. The command
\@ in front of a period specifies that this period terminates a
sentence even when it follows an uppercase letter.
"@
The additional space after periods can be disabled with the command
which tells LATEX not to insert more space after a
period than after ordinary character. This is very common in
non-English languages, except bibliographies. If you use
frenchspacing, the command \@ is not necessary.