Powdered graphite is recommended for cylinder locks (the type normally found on house front doors) which have small cylindrical pins in them that fit into small cylindrical holes. Wet lubricants tend to dry out or thicken over time and cause the pins to stick. Powdered graphite is exactly the same stuff as pencil lead, so if it's just one lock it is probably not worth buying it.
http://www.ebay.co.uk/itm/Silca-Grap...gAAOSwG-1WzcL0 You can make your own by rubbing a pencil on a nail file or similar. The advantage of the bottle is that you can puff the graphite into the lock and lubricate the pins rather than just the key.