This is an example of so-called thinnet or, specifically in the Ethernet context, 10Base2 cable. The name thinnet stems from the fact that it is much thinner than the original coaxial cable used for networking. The name 10Base2 referred to the fact that the cable was used to carry baseband signals at 10Mbps over distances of up to 200m.
In order to connect computers to such cables, BNC connectors were used. In the picture below three such connectors are shown: a T-connector, a male connector crimped to a cable and a 50Ω terminator. The matchstick is included to give a sense of size.
In normal use cable would be installed with male connectors at any point where it may be necessary to link a node to the network. If no node was present, the two ends of the cable could be joined by using a female-to-female connector to join the two male ends. However, if a node was present the connection would be made with a T connector as shown below. A fly lead could then be used to connect this new point with the female connector on the network card of the node.
Today these connectors are not often seen in real networks and a probably most useful the create BNC art. Below is a piece by me, entitled 7 Tees terminated twice.
