A HUB forwards all packets out all interfaces, except the one the packet was received on. This means information sent from Host A to Host B, is very easily intercepted by Host C. Even if Host C wasn't addressed in the data header. A HUB is considered a layer 1 (Physical Layer) device in the OSI Model.
An Ethernet hub, active hub, network hub, repeater hub, multiport repeater, or simply hub is a network hardware device for connecting multiple Ethernet devices together and making them act as a single network segment. It has multiple input/output (I/O) ports, in which a signal introduced at the input of any port appears at the output of every port except the original incoming. A hub works at the physical layer (layer 1) of the OSI model. A repeater hub also participates in collision detection, forwarding a jam signal to all ports if it detects a collision. In addition to standard 8P8C ("RJ45") ports, some hubs may also come with a BNC or an Attachment Unit Interface (AUI) connector to allow connection to legacy 10BASE2 or 10BASE5 network segments.
Hubs are now largely obsolete, having been replaced by network switches except in very old installations or specialized applications. As of 2011, connecting network segments by repeaters or hubs is deprecated by IEEE 802.3.