What's the difference of dmesg output and /var/log/messages?

dmesg prints the contents of the ring buffer. This information is also sent in real time to syslogd or klogd, when they are running, and ends up in /var/log/messages; when dmesg is most useful is in capturing boot-time messages from before syslogd and/or klogd started, so that they will be properly logged.

That depends on the operating system. For example on Solaris, dmesg is simply a shell script showing the last 200 lines of the /var/adm/messages.* files.

• We can say that dmesg is the subset of /var/log/messages and is maintained in ring buffer.
• /var/log/messages includes all the system messages including from starting of the system along with the messages in dmesg. In a nutshell logs from dmesg are dumped in /var/log/messages.
• /var/log/messages maintain the general system activity logs and dmesg maintains only the kernel logs.