Bridges are devices used to connect two networks using the same protocols.
Gateways are like bridges, they connect two networks together, however if the networks transmit data using different protocols they cannot transmit data to each other unless a gateway is used to connect them. A gateway acts as a translator for data passing through networks using different protocols. A message destined for a node outside its network would pass through the gateway, a gateway would read the message and convert the message into a protocol suitable for the outside network then transmit the message.