\include{file}
The \include command is used in conjunction with the \includeonly
command for selective inclusion of files. The file argument is the
first name of a file, denoting FILE.TEX. If file is one the file
names in the file list of the \includeonly command or if there is no
\includeonly command, the \include command is equivalent to
\clearpage \input{file} \clearpage
except that if the file FILE.TEX does not exist, then a warning
message rather than an error is produced. If the file is not in the
file list, the \include command is equivalent to
\clearpage.
The \include command may not appear in the preamble or in a file read by another \include command.
\includeonly{file_list}
The \includeonly command controls which files will be read in by an
\include command. It can only appear in the preamble.
\input{file}
The \input command causes the indicated file to be read and processed,
exactly as if its contents had been inserted in the current file at
that point. The file name may be a complete file name with extension
or just a first name, in which case the file FILE.TEX is used.