What is an electric charge?

Many school definitions explain electrical charge, for example:
- Electric charge interacts through electromagnetic fields.
- Electric charge is a force in an electric field and in a magnetic field if it moves.
- Electric charge is a property of matter related to the loss of neutrality of a substance.
- The electric charge constitutes an essential property of the elementary particles subjected to the electromagnetic interaction.
- Electric charge is something that causes the object that has it to experience a force.
- etc.
All these definitions of electric charge tell us nothing about the real nature of this charge. This is normal because we cannot understand the electric charge without understanding the notion of "field".
The field is also an abstract notion that our brain cannot imagine because it did not need it during its evolution. Moreover it is a quantum concept of which it is very difficult to make a real image of it. Although we don't have the sensory sensors to imagine them, the fields are very real.
Since the fields are invisible, the best image is of iron filings drawing curved lines around a magnet. These lines show us that an attractive or repulsive force is capable of acting at a distance in a portion of space called a "field".
Fields carry the energy of everything in the universe, from atoms to large galactic structures. Magnetism, gravitation, nuclear force, light and many other physical phenomena are carried by fields.

To understand the notion of charge, it is useless to go into extremely complex explanations of bosons and fermions, electrons and holes, virtual particles and moving charges.
Electric charges are the origin of the electrostatic forces observed between objects.
The value of an elementary charge “e” (i.e. the smallest) corresponds to the charge of a proton or an electron having a value of 1.60217653 × 10^-19 coulomb. The electric charge is therefore a scalar quantity carried by the electromagnetic field. However, ordinary matter tends towards electrical neutrality.
Electric charge interacts with the electromagnetic field like mass with the gravitational field.
These two interactions (electromagnetic and gravitational) are part of the four fundamental forces of nature with the strong and weak nuclear interactions.
In a gravitational field, mass flows along gravitational gradients (slopes).
In an electromagnetic field, charge flows along electromagnetic (potential) gradients.