Is electricity wasted when you've connected, say a mobile charger but are not using it?
Yes, some electricity is consumed. A charger is a relatively complex device that has multiple "circuits" internally. Some of these circuits dissipate power regardless of whether anything is connected to the charger's output.

The amount of power is miniscule, however &mdash; on the order of tens to hundreds of milliwatts. You'd have to leave it plugged in for a year before it consumed a single kilowatt-hour of energy. In other words, each charger you have plugged in is adding about a penny to your monthly electric bill.