a basic electrical circuit

Electricity
Electricity is a type of energy that can build up in one place or flow from one place to another. Electricity makes all kinds of machines work. It can have not only great power to move trains but also low power to run mp3 players and calculators.
Most machines you use at home, such as televisions and vacuum cleaners, use “current electricity.” This is the kind of electricity that comes from batteries and power points in your home. There is another kind of electricity, called “static electricity.” You have experienced this when someone “shocks” you after walking across a carpet floor.

A basic electrical circuit
When you flip a light switch on and off, you are closing and opening a circuit. A circuit is the path that electricity follows. For electrons to travel (creating an electric current), the circuit must be closed. When you flip the light switch off, you are opening the circuit and the lights turn off. When you flip the switch on, the circuit it closed and the lights come on. Closing the switch completes a circuit. Electricity flows from one battery terminal, through the wires to the switch and the LED(or load), and back to the other terminal.

LED
What is LED(light emitting diode)?
An electronic device that lights up when electricity is passed through it. When you connect power to LED, you have make sure positive lead(long leg) should connect to positive(+) and negative lead(short leg) should connect to negative(-).

series vs. parallel

A basic electrical circuit consisting of just one battery and one load resistance(ex. LED) are very simple to analyze, but the basic circuit is not often found in practical applications. Usually, we find circuits where more than two components are connected together.

Series Circuits
The basic idea of a "series" connection is that components are connected end-to-end in a line to form a single path for electrons to flow. A series circuit allows electrons to follow only one path. The loads(LEDs, resisters) in a series circuit must share the available voltage. In other words, each load in a series circuit will use up some portion of the voltage, leaving less for the next load in the circuit. This means that the light, heat, or sound given off by the device will be reduced.

Parallel Circuits
The basic idea of a "parallel" connection, on the other hand, is that all components are connected across each other's leads. In parallel circuits, the electric current can follow more than one path to return to the source, so it splits up among all the available paths. Across all the paths in a parallel circuit the voltage is the same, so each device will produce its full output.

In a series circuit, when one of the bulbs or one of the wires is left open or is broken, the entire circuit ceases. The break opens the circuit. Less expensive Christmas lights are usually of this type, and you have to search for the defective bulb. A parallel circuit is designed so that if one branch is defective, the flow of electricity will not be broken to the other branches.

open&close circuit and conductor

Electrons always have a negative ("-") electric charge, and it is this electric charge that is responsible for what we call electricity. When electrons move from one place to another they take that negative charge with them. It is this movement of charge that we refer to as electricity!
This movement or flow of electrons is called an electric current. We often find electrical currents inside wires that conduct electricity. Conductors are materials that contain many freely moving electrical charges (like copper, silver, aluminum, and gold). Insulators are materials that contain few moveable electric charges (like plastic, glass, rubber, and air).

An electrical current traveling through a wire. Electrons don't really race from one end of the wire to the other like sprinters at a track meet. In an electric current the electrons jump from one atom to another, pushing other electrons in front of them as they go. They really behave more like runners in a relay race, passing along the baton (electrical energy) from one atom to the next. The electrons don't actually travel very fast, but the electrical energy that they create travels at the speed of light!

Electrons move along a path called a circuit if a "pushing" force (a source of electricity) is part of the circuit. Sources include things like batteries, wall outlets, and solar panels.
While the electrons are moving through the conductors, the circuit is complete, or "closed." If any part of the circuit is removed, the electrons stop and the circuit is "open." You've probably opened and closed circuits before without ever realizing it: When you turn the lights on or off, you are closing or opening a circuit!

reference : The NASA Sci Files, Dr. D's Lab