Physical layer (1Layer) the actual data is in "bits" ( 0 and 1)
When it reaches to data link layer or Layer 2 it becomes "frame" with source and destination mac address getting added.
When it reaches the 3rd layer or the networking layer it becomes a "packet" with source and destination IP address attached to it.
Finally it become "segment" at Layer 4 or TCP/IP (sercure) layer. Basically "segment" term is used for TCP connections and "datagrams" for UDP.
So what about other layers like : session, presentation and application ? In these layers the data is termed as "PDU" or "protocol data unit".
So when someone says, that a "frame" is being received by a networking device, then you know that the device is switch.
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time and order are not guaranteed. A datagram consists of header and data areas, where the header contains information sufficient for routing from the originating equipment to the destination without relying on prior exchanges between the equipment and the network. The source and destination addresses as well as a type field are found in the header of a datagram.
A packet consists of two kinds of data: control information and user data (also known as payload). The control information provides data the network needs to deliver the user data, for example: source and destination addresses, error detection codes like checksums, and sequencing information. Typically, control information is found in packet headers and trailers, with user data in between.
In computer networking and telecommunication, a frame is a digital data transmission unit or data packet that includes frame synchronization, i.e. a sequence of bits or symbols making it possible for the receiver to detect the beginning and end of the packet in the stream of symbols or bits. If a receiver is connected to the system in the middle of a frame transmission, it ignores the data until it detects a new frame synchronization sequence.
