A segment is the Layer PDU for TCP. UDP uses "datagram" instead.
TCP segment contents:
- Source/Destination ports: app identifiers
- Sequence number: byte position
- Acknowledgment number: what's received
- Flags: SYN, ACK, FIN, RST
- Window size: flow control
- Checksum: error detection
- Data: payload
The Maximum Segment Size (MSS) is typically bytes for Ethernet.