In most computer systems, a byte is a unit of data that is eight binary digits long. A byte is the unit most computers use to represent a character such as a letter, number, or typographic symbol (for example, "g", "5", or "?"). A byte can also hold a string of bits that need to be used in some larger unit for application purposes (for example, the stream of bits that constitute a visual image for a program that displays images or the string of bits that constitutes the machine code of a computer program).
Packet switching is a communications paradigm in which packets (units of information carriage) are routed between nodes over data links shared with other traffic. In each network node, packets are queued or buffered, resulting in variable delay. This contrasts with the other principal paradigm, circuit switching, which sets up limited number of constant bit rate and constant delay connections between the nodes for their exclusive use for the duration of the communication. When a unit of data, beginning at the application layer (top layer) is transmitted to lower layers, headers and trailers are appended to it. Once the unit of data reaches the IP layer and source and destination IP addresses are added to it, it then is known as an IP datagram. The unit of data, feeling a little bloated from having all this extra baggage is then sent to the link layer where source and destination MAC addresses are perpended to it. Now it is called a frame - and more accurately an Ethernet frame (if the physical layer is Ethernet). To be precise, the unit of data passed between the IP layer and the network interface card is really a packet of data. Thus, a packet can be an entire IP datagram or perhaps a fragment of an IP datagram.
In computer networking, the maximum transmission unit (MTU) of a communications protocol of a layer is the size (in bytes) of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface (NIC, serial port, etc.). Standards (Ethernet, for example) can fix the size of an MTU; or systems (such as point-to-point serial links) may decide MTU at connect time.
