With Ethernet, broadcasts are specialized frames that are destined for all devices on an Ethernet network. Broadcasts use a MAC address of FF-FF-FF-FF-FF-FF. This is a special MAC address because it is the highest number allowed in the 48-bit schema of MAC addresses. In binary, all 48 bits are set to 1.
Multicasts are specialized broadcasts. Multicasts are used by higher layer protocols to direct traffic to more than one select destination, rather than a broadcast, which is sent to all destinations. Application layer multicasts start with 01-00-5E prefix. The rest of the digits are assigned by the application layer protocol handling the multicast. However, other Layer 2-only multicast addresses do not have the prefix of 01-00-5E; for example, STP with MAC address of 01-00-0c-cc-cc-cd. For the most part, Ethernet networks treat multicasts like broadcasts by default. Several higher layer protocols, such as IGMP (Internet Group Messaging Protocol), can be used by switches to differentiate the traffic and forward only multicast out specific ports.