Voltmeter Measures voltage. The proper name for voltage is potential difference but voltage is more widely used.
Relay An electrically operated switch, for example a 9V battery circuit connected to the coil can switch an AC mains circuit. The rectangle represents the coil. NO = Normally Open, COM = Common, NC = Normally Closed.
Aerial (Antenna) A device to receive or transmit radio signals. It is also known as an antenna.
Logic gates process signals which represent true (1, high, +Vs, on) or false (0, low, 0V, off). For more information please see the page on logic gates. There are two sets of symbols: traditional and IEC (International Electrotechnical Commission).