dmesg displays the system message buffer. It's output is just like /var/run/dmesg.boot, although dmesg displays things that occur after boot as well. It can be helpful in tracking down problems that might not otherwise be noticed.
> dmesg [snip] ad0: 194481MB <Maxtor 6B200R0/BAH41BM0> [395136/16/63] at ata0-master UDMA133 ad1: 194481MB <Maxtor 6B200P0/BAH41BM0> [395136/16/63] at ata0-slave UDMA133 acd0: DVDR <SONY DVD RW DRU-500A/2.1a> at ata1-slave PIO4 ad1: WARNING - WRITE_DMA UDMA ICRC error (retrying request) LBA=3205439
> dmesg [snip] Mounting root from ufs:/dev/ad0s1a pid 6152 (audacity), uid 1001: exited on signal 11 (core dumped)