Iftop

iftop is a free software command-line system monitor tool that produces a frequently updated list of network connections. By default, the connections are ordered by bandwidth usage, with only the "top" bandwidth consumers shown. 
The iftop website gives the following description: "iftop does for network usage what top(1) does for CPU usage. It listens to network traffic on a named interface and displays a table of current bandwidth usage by pairs of hosts. Handy for answering the question 'why is our ADSL link so slow?'".
Description.
iftop monitors network traffic and displays a table of current bandwidth usage. An interface may be specified or, if not, it will listen on the first interface it finds which looks like an external interface (with libpcap and libncurses). iftop must be run with sufficient permissions to monitor all network traffic; on most systems this means that it must be run as a root user, see sudo.
By default, iftop will look up hostnames associated with addresses and counts all IP packets that pass through the filter. Hostname look-up can add substantial traffic, in and of itself, and may result in an inaccurate display of network traffic. You may wish to suppress display of DNS traffic by using filter code such as "not port domain", or switch it off entirely, by using the codice_1 option or by pressing "n" when the program is running. Using the codice_2 option makes it possible to show packets entering and leaving a given network.