Grounding on the wooden floor Let's suppose that I'm touching a live wire and standing on a wooden floor. Will I be shocked? Or e.g. standing in a plastic tub filled with water. I cannot imagine that there is a closed circle to the ground (I mean literally the Earth's ground). How exactly should I imagine the ground? Is it always just Earth? Or can it be anything with 0V?
 A: 
How exactly should I imagine the ground? Is it always just Earth? Or can it be anything with 0V?

In general, as the other answers say, we can choose any location we like in a circuit or system, and call that our ground.
However in the case of the mains outlet, the power company has already chosen to use the literal earth as that reference. So when we say the hot line carries 120 or 240 V AC, it is the earth ground that they are referencing the instantaneous voltage to. A 120 V (rms) AC circuit will be oscillating between positive and negative 169 V relative to earth.
So in your example, I'd strongly suggest anyone analyzing it also use the earth as their reference.

Let's suppose that I'm touching a live wire and standing on a wooden floor. Will I be shocked? Or e.g. standing in a plastic tub filled with water. I cannot imagine that there is a closed circle to the ground

Remember that AC current can also pass through capacitive paths, not just resistive ones. So if you are standing on a wooden floor, and a grounded water or gas pipe is running under the floor beneath you, or if your plastic tub is sitting just above a concrete foundation, there may be enough capacitance between you and the earth to carry a dangerous current through you to ground.
Definitely don't experiment with this scenario.
A: No, you will not get an electric shock, but I strongly advise against trying it.
Electric potential is a relative measure. There is nothing with "0V"; all you can say is that one terminal has 0V potential difference with another. Hence, in order to feel the effects you need a closed circuit. A (perfect) insulator breaks that circuit, and hence you will be safe. And, it's not the voltage you feel, it's the current. As the wooden floor puts a very high resistance in the circuit, the current through you will be very small, so you do not feel it.
However, and here is why I don't recommend it, no insulator is ever perfect. For example, your wooden floor may be wet, and hence conduct electricity. Then you will feel the shock. There may also be other factors involved that are unknown to you and that could make the difference between life and death for you. Unless you really understand everything involved, it's best to play it safe and turn the circuit off before touching it.
A: There is 'ground' as any chosen $'0V'$ reference. There is also 'ground' used as an infinite store or supply of charge, i.e. it is considered in the use context to be able to absorb infinite charge, or supply infinite charge (Wikipedia). It need not be the physical earth.
While wood is a bad conductor of electricity (ref), charge can gather on it (ref), meaning that, if the wire that you say you are touching is charged opposite to the charge on the floor that you are standing on, you could get a shock as the charge travels through you to the floor, or in the opposite direction.
