This had been discussed many & many& ... times. pfffff

Ethereal is a sniffer!
It shows every single packet going through your NIC (ethernet layer).

So the question u should ask yourself is what type of flows should normaly go through the interface where your PC is connected.
- If the network device is a Hub you should see all media traffic
- if the network device is an ethernet switch you should see only broadcast and unicast traffic addressed to your PC
- if the network device is a router you should see only unicast traffic addressed to your PC
- if the network device is a modem see only unicast traffic addressed to your PC
-.....

There is doc u can read:
- search for IEEE 802.1d standard
- search for CSMA-CD
- search for IEEE 802.3
....
- or search for smth like "Ethernet Network for dummies"