Broadcast

A broadcast is likened to radio and is by definition one sender and multiple receivers. Ethernet can broadcast, IP can broadcast.

Broadcasting is stateless, meaning you cannot use TCP with it. Most broadcasts are done with datagrams.

Broadcasting is similar to multicasting and different to unicasting which has one sender and one receiver only (served over protocols such as TCP).

Wifi

Wireless LAN by definition always broadcasts. The only thing preventing listening to all traffic by default is the MAC address filter. As wireless technology progresses you will see a lot more programs making use of broadcasting or multicasting. Cryptography coupled with broadcasting will ensure what hosts on the wireless LAN you'll be talking to.

Exploitation of Broadcasts

In the Internet broadcasts are also possible but they are restricted to a specific site. At one point this wasn't so and an exploit called smurf became annoying especially for IRC networks. Smurf by Tfreak exploited the fact that one could spoof the source address of an ICMP echo packet destined for the broadcast address of a large network. All the computers on that network would reply with ICMP echo reply packets to the spoofed source which then becomes a victim to DoS. Smurf is also properly called an amplified broadcast attack. Router manufacturers soon disabled the option that a router should direct broadcasts inside a LAN from the Internet, this was done so because it would have been a lot harder to convince ISP's to filter spoofed source packets from their network (egress filtering).