Amplifier (general symbol) An amplifier circuit with one input. Really this is a block diagram symbol because it represents a circuit rather than just one component.
An OR gate can have two or more inputs. The output of an OR gate is true when at least one of its inputs is true.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.
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).