Linux.com

"A Linux machine has a logging system which keeps track of what everything is doing. Anytime you authorize with sudo it gets logged. Anytime you (or someone else) connects via ssh it gets logged. Apache logs connections, mail servers log emails sent and refused. Pretty much everything keeps a log of what it is doing so you can later troubleshoot it or simply have a record of it.For those that are security minded it may not be a bad idea to keep duplicate of your logs by sending them not only to the local machine but to a remote machine as well .... "