A frame is the Layer PDU wrapping the packet with link-layer addressing.
Ethernet frame contents:
- Preamble: synchronization
- Destination/Source MAC: -bit addresses
- EtherType: payload protocol
- Payload: the packet (- bytes)
- FCS: error detection
Frames travel within one segment. Crossing a router strips the old frame and creates a new one with new MACs. The packet stays the same.