Ethernet traffic capture

This depends on your network. If you have a central Hub (not a Switch) you can see all data passing by, but it is highly unlikely that you're not using a Switch (because hubs are slooooooow). On a switch, you can only receive broadcasts and other single packets that are flooded to all ports because of an unknown destination MAC address, but you will not see complete communication flows of others. The switch will hide it from you because it is connecting stations directly without broadcasting the data to all other stations.

One way to get around this problem of not seeing the packets you want is to use a SPAN session (a.k.a mirror port, monitor port etc.), but that requires the switch in use to be manageable. Some better SOHO switches can do that, usually via web interface. If your switch is "dumb" and cannot be managed you're out of luck, unless you try to leverage some hacking techniques like ARP cache poisoning on the network. Or you can get one of the fancy Dualcomm switches with built-in monitor ports and replace your switch with it :-)