Sahir's right. Every packet sent onto an Ethernet segment is seen by all the stations. However every station ignores it except the one whose MAC address matches the destination MAC address in the packet. This is unicast. With broadcast, the packet is given a special MAC address (all the FF's) which tells every station to accept and process it regardless. So there's just one packet sent but everyone pays attention. Analogy - Go into a crowded shop and call "Fred" and only Fred will turn round. But yell "Fire" and everyone will pay attention. Multicast is similar except it's the multicast group rather than all the stations that pay attention to the packet.