If you want to find all files that have changed in the last day you can use the command
find / -mtime -1
If you run a server and want to monitor the file system and get a daily email with the list of files that were changed in the last 24 hours you can just add this as a cron job. The following example added to your crontab will scan your /var/www folder and send an email to email@example.com.
minute (0-59), # | hour (0-23), # | | day of the month (1-31), # | | | month of the year (1-12), # | | | | day of the week (0-6 with 0=Sunday). # | | | | | commands 0 0 * * * find /var/www/ -mtime -1 | /usr/bin/mail -s "list of changed files" firstname.lastname@example.org
Adjust as needed.