Free software (FSF definition) is software that is distributed in a manner that allows its users to run the software for any purpose, to redistribute copies of, and to examine, study, and modify, the source code. The term was coined in 1983, with "free" denoting the broad freedom given to users, rather than software that is free of charge (which is freeware). Open-source is not always free, though.
The functionality Wireshark provides is very similar to tcpdump, but it has a GUIfront-end, and many more information sorting and filtering options. It allows the user to see all traffic being passed over the network (usually an Ethernet network, but Wireshark also supports decoding 802.11 frames and even USB traffic) by putting the network card into promiscuous mode.