The figure shows a single computer that has multiple web pages open along with an instant messaging window, an e-mail application, a streaming video application and a VoIP phone application running.  There is a series of data blocks of varying colors that represent the various applications running on the computer.  The series of blocks are created by the transport layer breaking the data into segments, and each block is a segment.  This is multiplexing where multiple applications can use the network at the same time.  Each data block can have error checking performed on it to check if the segment was changed during transmission.