I am rather new to electronics, and I am reading up on the topic. I came across the term ground. I get the large scale applications like the wires leading into the ground from power lines, but how is ground applied to smaller, simpler circuits? Imagine a circuit with a battery and an LED. Wires lead out of the positive and negative side of the battery and connect to the points on the LED. Where would the ground be on that circuit?
Traditionally, "ground" is the lowest potential in a circuit, e.g. the minus side of a battery or DC supply. In an AC circuit, such as the 110 volts AC coming into your house (at least in the US), ground will be the wire closest to the earth potential (i.e. you can usually touch the white wire, which is the ground or neutral wire, but get a bad shock if you touch the black "hot" wire -- but don't count on these conventions, as the wiring may be incorrect and/or there may be a ground fault in your wiring).
Below, in the simple DC circuit on the left, there is an explicit wire connecting the ground lead from the battery to the minus side of the LED. In the circuit on the right, ground symbols (the open triangles) are used instead; in a large circuit there will be ground symbols scattered all over a schematic, all assumed to be connected together. (Sometimes other but similar symbols are used for ground.) These two circuits are identical.
The one exception to the ground being the lowest potential in a circuit is when you have both positive and negative supplies, such as in the following circuit:
Here there are two batteries, so you have both a +6v supply and a -6v supply, relative to the ground lead between them. The reason for the two supplies is the comparator, which needs both positive and negative supply voltages. This means the output of the comparator will also swing both positive and negative. Op-amps are another type of IC that sometimes require both positive and negative supplies. However it is more common for both op-amps and comparators these days to require only a positive supply.
Sometimes there may be more than one ground is a circuit; this is typical in circuits that have both digital and analog circuitry, and is done to minimize noise. There will be only one point on the circuit where the two grounds are connected. Another way that noise is mitigated on printed circuit boards (PCBs) is to have an entire layer of the board dedicated to being a ground plane. This makes it easier to connect all the pins and other components of the circuit that need a ground by just dropping a via down to the ground plane. If a ground plane is not used, traces have to be routed all over the board to connect all the grounds together.
"Ground" is a much abused term in electricity and electronics. Occasionally, particularly in AC power distribution and some radio antenna systems, it does mean "a connection to the earth".
However, in most electronics, particularly portable battery-operated devices, "ground" is just the point in the circuit that the designer chose to call "zero volts" and to use as a reference when measuring voltages elsewhere in the circuit. I think it would be better to call it "reference" or "common", but we seem to be stuck with misusing "ground".
In most circuits these days, "ground/reference" is the most negative terminal of the power supply, so voltages elsewhere are positive relative to "ground".
In some cases, the positive terminal of the power supply is "ground", and voltages elsewhere are therefore negative (there's one logic family that is supposed to run from a -5.2 volt suppply).
In many analog circuits, particularly those using op-amps, "ground" is the middle of the power supply, so we have both negative and positive voltages in the circuit.
To put it simply, wherever you like. The only requirement for a ground is that everything must pick the same ground in order to avoid unwanted current flows between different systems. The negative supply is the most common choice, but no more or less valid than any other.
You seem to be confusing (as I did) ground with protective earth. "Ground" is the reference point for the voltage potentials in the circuit, is designated 0V, and all other voltages are compared to that. Consider this circuit, where power is provided from two 9V DC batteries in series:
Any of the points A, B, and C may be considered ground but the circuit will be considered to have different voltages depending on which point is selected. If point A is ground, B is -9V, and C is -18V; if B is ground, A is 9V, and B is -9V; if C is ground, A is 18V, and B is 9V.
Protective earth, on the other hand, is used in high-voltage AC circuits to provide a safe path for unexpected current, protecting people and property from electrocution, failure, and fire.
Unfortunately, both "ground" and "protective earth" are often referred to simply as "ground" (especially, in my experience, by people who work exclusively with one or the other but not both).
Caveat: I'm new at this too, and hoping to learn from this answer as well. If there's anything I've missed (or I'm just plain wrong), please don't hesitate to comment and/or edit! Thanks.
As others have said, the Ground reference in a circuit is a Relative point; i.e. Voltages are defined in that circuit with respect to the chosen Ground reference.
Historically Germanium Transistors were mostly available as PNP devices. In these circuits a Negative supply was the norm with Ground being chosen as the +Ve potential.
The circuit below is from a classic Guitar effect unit (source)
Notice the +Ve of the battery is labelled as Ground and the power is Negative.
But this ground scheme may be incompatible with other Guitar Effect units....
So it can be redrawn: