Working with ESX(i) log files is important when troubleshooting issues within the virtual environment. So which information is stored where? Below is a list of the log files and what they contain. Note that all of them are located in /var/log.

auth.log	ESXi Shell authentication success and failure
dhclient.log	DHCP Client logs
esxupdate.log	ESXi patches and update installation logs
hostd.log	Host management service logs, including VM and host tasks and evens, vpxa related events.
shell.log	ESXi shell usage (history of all commands run on the command line)
sysboot.log	VMkernel startup and module loading
boot.gz	Boot log information. Read this via zcat command.
syslog.log	Management service initialization, DCUI use, scheduled tasks.
usb.log	USB device arbitration events like discovery.
vob.log	VMkernel observation events
vmkernel.log	Core VMkernel logs, including storage and networking events, VM startup
vmkwarning.log	VMkernel warning and alert log messages
vmksummary.log	A summary of ESXi startup and shutdown. It also records hourly heartbeat with uptime, number of VMs running and service resource consumption.