HP Procurve uses the same command, IP helper-address 111.222.333.444 as Ron mentioned is used for Cisco IoS. Cisco SMB switches like the SG300 use ip dhcp relay address 111.222.333.444. The list is endless really. That's not my purpose. I just wanted to add the "why" you need to use this command if you want to use one DHCP server for multiple VLANs/subnets.
Since a layer 2 network is broadcast domain the DHCP request from a computer normally will stay within its VLAN because it's a UDP broadcast. Broadcast, as you may know, can't traverse a layer 3 boundary (a router or layer 3 switches). By adding the IP helper-address command at the gateway the layer 3 device can forward the UDP request from the computer for DHCP to the server (and other protocols such as TFTP(port 69), DNS (port 53), BOOTP (port 67, 68), TACACS (port 49), the older Time service (port 37, not NTP), NetBIOS (port 137, 138).