Why should a transistor circuit be grounded? The way I understand it is that ground is an infinite supply of electrons and so it is equivalent to the negative terminal of a battery. But in this page , there is both a negative terminal and the circuit is also grounded. If I've understood rightly, for a circuit to be complete, we need a positive terminal, some circuit elements & a negative terminal. If grounding is equivalent to a negative terminal, why do we need it in the circuit shown above? (By the way, all the circuits in the book I'm referring were not grounded until transistors started off. You could say that added to the confusion)
The term "ground" can mean different things. When a circuit is looked at locally, ground is simply the one net someone picked to call 0V so that all other voltages are understood to be relative to it. Voltage is after all a relative concept. There is no such thing as 20V absolute, only 20V here with respect to there. "Ground" is a handy short form for a common there so you don't have to keep saying it.
Such a ground is often chosen to be the negative supply voltage. That makes other voltages positive, which just makes things mentally easier. In general you try to chose ground as the point in the circuit signals and the like are referenced to by individual subsections. Usually there is a clear and obvious choice. Sometimes you just have to pick one.
The other "ground" refers to real earth potential, or at least the potential of the general surroundings. This matters when power and signals are coming from or going outside your little circuit. For power and saftey, you have to assume a user could be tied to this ground, and you have to make sure you don't have a dangerous potential to ground to avoid zapping someone. Earth ground can also matter for radio systems since some types of antennas actually use the earth as part of the overall antenna system.
From a circuit perspective, ground is relative - it is just a name for a wire (trace, net, node, etc) which other things may be measured or driven in reference to. As such it's largely arbitrary - for example, there are (or at least were) motor vehicles where the positive battery terminal rather than the negative was connected to the chassis ground. The addition of a ground symbol in your book at the same point in time when transistors were introduced is either a coincident change of style, or a general increase in the sophistication of the diagrams at a point when the complexity is increasing and thus making organizing principles such as the idea of a "ground node" more valuable.
In a communication circuit, especially a radio, "ground" may have some literal meaning or at the least implication of connection to an earth ground (potentially a conductive stake driven far into the ground, possibly with the surrounding soil chemically treated if it's not sufficiently conductive). But that may not be reliably true for a handheld walkie-talkie, and it's certainly not true for an aircraft radio installation - in fact the potential of the aircraft "ground" might become quite different from that of the ground "ground" during the flight.
But while in circuitry we think of "ground" as purely relative, in chemistry and physics, there is a concept of neutral charge which actually is absolute. A neutral charge is what you would have in an atom or molecule with an equal number of protons and electrons, or some larger unit of matter with equal net numbers. The absolute charge of an object determines the right-hand-rule force it would experience if moving through a magnetic field, while the charge relative to another nearby charge will determine the electric field (and resulting force) set up between the two.
Because we can also create an electric field between between two points in a circuit, it might be tempting to think of "ground" as having neutral charge, but ultimate there is nothing that says that must be the case. Indeed, relative to the atmosphere, the earth has negative charge, though I'm not finding a ready answer as to the absolute charge of earth, meaningful atmosphere, or the two taken together. Presumably we are slightly subject to the sun's magnetic field, so if the earth had a substantial enough net charge it would cause some orbital perturbation, but then the subject of solar magnetism is hugely complicated - including the field loops called sunspots which have their own more obvious effects on radio communications and long wires here on earth...
Chris provided a more complex answer, so I'll make simple one.
This confusion is manly a problem of English language using same name for different concepts. In say German, this strange new ground would be called mass and is a bit different than real ground.
Basically the mass is just a convenient node in the circuit. In theory it can be absolutely any node in the circuit (more on that in Chris's answer) but in practice, it's just a simpler way of showing the connection to negative terminal of the battery. For example take a look at this circuit. Here I connected battery negative terminal to ground an the diode cathode to the ground. That is same as this circuit where the cathode is explicitly connected to negative terminal of the battery.
Another thing which usually comes with the mass is the one terminal voltage source. Take a look at this circuit. It's same as the other two, but here we don't see the battery at all. The advantage of using this approach is that you're now getting into real electronics and you can expect more complex circuits. They can get very quickly very complicated to read, so the one terminal voltage source and the "ground" symbols are used to show that the component is connected to battery. For example in this circuit, it's completely irrelevant to the circuit itself how the voltage source looks like and how the emitter of the transistor is connected to the negative terminal.
Here are two funny pictures illustrating a gravitational ground...
... and an electrical ground.
Grouding and Earthning in wider sense are the same concepts.they are the concepts related to relative voltage.whenever we take something w.r.t 0 volts(null potential) it is referred to as grounding.for ex.lets consider 5V.It means negative plate is at 0V and positive is at 5V.This is w.r.t ground.And when we don't take w.r.t to ground it means that positive is at 2.5V and negative is at -2.5V which again makes it 2.5-(-2.5)=5V. Thus GROUNDING is nothing but the reference what we are taking w.r.t 0V.