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As Peter wrote, Metro Ethernet is a vast topic. However, from a non-provider perspective, the idea is to allow interconnecting widely physically separated (your) site networks just about as simply and similar to connecting your LAN together at one of your sites. I.e. you just connect different (your) site devices with an Ethernet connection, and you use that connection pretty much like you would any other Ethernet link between your devices.
From a carrier or MAN provider perspective, running a widely distributed, multiple customer Metro Ethernet (or a carrier Ethernet) network, you'll want some features not found in (as you say) "normal" Ethernet equipment.
What's different about Metro Ethernet switches is they generally offer features not found in "normal" Ethernet switches. Again, from a non-provider perspective, you often only need/desire a few additional features (and for that reason, it's not uncommon for non-providers to use "normal" Ethernet switches with Metro E - I've also seen Metro E switches used with "ordinary" LAN Ethernet networks). From a provider's (carrier's) perceptive, you'll really want more features. Because of these two perspectives, you'll often find Metro Ethernet switches marketed to non-providers, and Carrier Grade Ethernet switches marketed to the MAN Ethernet providers.
