Spanning-Tree Protocol (STP) as defined in the IEEE 802.1D is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For an Ethernet network to function properly, only one active path can exist between two stations. Loops occur in networks for a variety of reasons. The most common reason you find loops in networks is the result of a deliberate attempt to provide redundancy - in case one link or switch fails, another link or switch can take over.
A VPN is a service that offers secure, reliable connectivity over a shared public network infrastructure such as the Internet. VPNs maintain the same security and management policies as a private network. They are the most cost effective method of establishing a virtual point-to-point connection between remote users and an enterprise customer's network.
It is a logical area in a computer network where any computer connected to the network can directly transmit to any other computer in the domain without having to go through a routing device.
A bridge connects two different LAN networks. A switch is something like you can connect many computers to a switch and then one computer can connect to another through the switch. Switch is a unicast one to one connection
A router is a device or sometimes a software in a computer which decides the next network point to which a packet should be forwarded to reach its destination on Internet. It is usually included as part of the network switch and is located at a gateway, including each point-of-presence on the Internet. The router is connected to at least two networks and determines which way to send each information packet based on its understanding of the state of the networks it is connected to.