dmesg shows the kernel ring buffer containing low-level system messages.
Per default, dmesg shows a timestamp:

12:59:26 fnordomator ~ > dmesg | tail
[101925.211846] usb 2-1.1: USB disconnect, device number 16
[110486.855788] usb 2-1.1: new high-speed USB device number 17 using ehci_hcd

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If you're a human, use dmesg -T to print the timestamp human readable:

12:59:31 fnordomator ~ > dmesg -T | tail
[Di Apr 21 12:43:16 2015] usb 2-1.1: USB disconnect, device number 16
[Di Apr 21 15:05:57 2015] usb 2-1.1: new high-speed USB device number 17 using ehci_hcd