What is quantum physics, actually?
Quantum physics is a field of physics that deals with nanoscale phenomena, that is, effects concerning individual molecules, atoms, or electrons. Behaviour of these particles differs greatly from what we are used to in our macroscopic world and that is the reason that we need special physical theory to describe such objects. The name itself — quantum physics, or quantum mechanics — stems from the weirdness of the nanoscopic world. Some quantities describing such systems (such as energy or momentum) can only change in discrete amounts (quanta) and not continuously like in classical physics (that is, physics dealing with the macroscopic world we live in).
What are the strange effects that can occur?
The first thing I already mentioned is the quantisation of some physical quantities. An atom, for instance, cannot absorb or emit an arbitrary amount of energy but only some very specific energy ‘packets’. This fact lies at the foundations of spectroscopy which can serve to identify atoms or molecules based on the spectrum of the light they emit.
What is next?
Another well-known quantum effect is the wave-particle duality. Already in the 17th century, scientists debated whether light is a stream of particles or a wave. (The most prominent supporter of the former idea was Isaac Newton, the latter was the proposal of Christiaan Huygens.) At the beginning of the 20th century, it became clear, that light behaves as both particle and wave depending on the phenomenon we want to describe. Propagation of light is best described assuming it is a wave, while absorption and emission happen only in discrete particle-like amounts which is connected to the quantisation of energy.
Similar behaviour can also be seen with atoms, electrons, and other particles. This was first proposed by Louis de Broglie — if light (a wave) can behave like a particle, particles should also have wave-like properties. The wave-like nature of electrons, atoms, and molecules has been experimentally tested so we know this really happens.
Quantisation and wave-particle duality. Is that all?
No. The last major difference from classical physics is the superposition principle. It states that a particle can be in a superposition of different values of a single quantity. An atom can thus be at several places at once.
That is impossible! Surely, scientists cannot measure one atom at several places at once.
That is partly true. When you measure the position of an atom, you will always find it at one specific place and not at two places simultaneously. But between two measurements, we cannot really talk about the atom’s position. All we can describe is the probability that the atom will be at a certain position, nothing more. It is actually connected to the wave-particle duality — since the atom travels as a wave, its position during this time is not clearly defined. It is similar to a wave travelling on a piece of string which also does not have a particular position but takes up a finite space.