The NVRAM is the Non-Volatile RAM. It stores all kind of parameters. You can check the with the command nvram show | sort. The output should be:
aa0=3 ag0=255 boardflags2=0 boardflags=0x0118 boardnum=42 boardrev=0x10 boardtype=0x0708 boot_ver=v3.5 boot_wait=off bootnv_ver=6 ccode=0 cctl=0 clkfreq=216 dl_ram_addr=a0001000 dmz_ifname=vlan2 dmz_ifnames=vlan2 dmz_ipaddr=192.168.20.1 dmz_netmask=255.255.255.0 dmz_proto=static eou_device_id=3K2UWR55 eou_private_key=**secret** eou_public_key=**secret** et0macaddr=??:??:??:??:??:?? et0mdcport=0 et0phyaddr=30 free_ifname=vlan4 free_ifnames=vlan4 free_ipaddr=192.168.40.1 free_netmask=255.255.255.0 free_proto=static gpio5=robo_reset ifup_interfaces=wan lan dmz voip free il0macaddr=??:??:??:??:??:?? lan_dns=192.168.10.10 lan_gateway=10.0.0.10 lan_ifname=vlan1 lan_ifnames=vlan0 eth1 eth2 lan_ipaddr=192.168.10.10 lan_netmask=255.255.255.0 lan_proto=static log_ipaddr=192.168.10.1 os_flash_addr=bfc40000 os_ram_addr=80001000 pa0b0=0x15eb pa0b1=0xfa82 pa0b2=0xfe66 pa0itssit=62 pa0maxpwr=0x4e pmon_ver=CFE 3.91.7.0 scratch=a0180000 sdram_config=0x0062 sdram_init=0x010b sdram_ncdl=0xfe0007 sdram_refresh=0x0000 sromrev=2 vlan0hwname=et0 vlan0ports=0 5 vlan1hwname=et0 vlan1ports=1 5 vlan2hwname=et0 vlan2ports=2 5 vlan3hwname=et0 vlan3ports=3 5 vlan4hwname=et0 vlan4ports=4 5 voip_ifname=vlan3 voip_ifnames=vlan3 voip_ipaddr=192.168.30.1 voip_netmask=255.255.255.0 voip_proto=static wan_device=eth0 wan_ifname=vlan0 wan_ifnames=vlan0 wan_ipaddr=10.0.0.7 wan_netmask=255.255.255.0 wan_proto=static watchdog=5000 wl0_akm=none wl0_channel=0 wl0_closed=0 wl0_crypto=tkip wl0_ifname=eth1 wl0_infra=1 wl0_mode=ap wl0_radio=0 wl0_ssid=OpenWrt wl0_wep=disabled wl0gpio2=0 wl0gpio3=0 wl0id=0x4320