The way I describe the filters is that the broadband signal is not filtered & runs straight through the item.
The filter bit is to stop any erronious electrical noise generated or picked up by any piece of equipment you have plugged in from being dumped on the network which the broadband signal might overhear
Imagine 2 people in a room having a conversation, (ones like the exchange, the other like your router). If there's someone with a radio blearing out in the same room (this is like a phone, answer machine sky box or anything you may have plugged in your phone sockets) it makes the conversation difficult or impossible.
If you stick the offender with radio in another room, you're filtering the noise out so the conversation can be maintained. A Microfilter is like putting your other telephony equipment in the other room.
The full dsl face plate is the same thing but it also filters out the internal wiring beyond your master socket ,in case thats a problem