Making no guarantee about whether data will reach its destination, it just decides where the data should be sent. The third, the transport layer, provides data flows for the application layer. It is at the transport layer where guarantees of reliability may be made. The fourth, the application layer, is where users typically interact with the network. This is where telnet, ftp, email, IRC, etc. reside.
Packets are the basic unit of transmission on the Internet.
They contain both data and header information. Simply put, headers generally consist of some combination of checksums, protocol identifiers, destination and source addresses, and state information. Each layer may add its own header information, so it can interpret the data the lower layer is handing it. In Figure 2, we see a sample Ethernet frame. This is the product of a packet which has gone from that application layer all the way to the link layer.