When it comes to professional printing, it is important that you have a something called a bleed and that all your important content are within the safe zone.
This comes about because of the nature of the printing process itself.
Commercial printers often print on massive runs (hence why printing 250 business cards costs relatively so little compared to doing it yourself – especially when you factor in time spent, printer ink, cost of the paper etc).
Due to the fact that printing right to the edge of the paper is extremely difficult (mainly due to the fact the getting the paper positioned accurately in the printer is difficult). To get around this, commercial printers will print business cards (and other business stationery) on larger paper than necessary and will print each card/stationery item 6mm larger than the final product will be when finished.
The cards/stationery are then cut down after the print run is complete on large cutting presses down the proper size of 85mm by 55mm (for cards that is, letterheads etc are cropped to their proper dimension).
So the Bleed is merely there to ensure that the design stretches all the way across the edge and to give the cutting machine a tiny margin of error.
So they the “Safe Zone” Then?
Commercial printers often emphasis a safe zone of 3mm. Due to the nature of the cropping machines, they may “drift” a little during the print run which means that the artwork may not be perfectly centred in the cutter when the cut is made. The 3mm safe zone is to safeguard against this and ensure that all the important details (such as your contact details) aren’t cut or cropped.
Without the 3mm safe zone, the card details could be cut off, leaving you with a potentially useless card.