It is a fundamental particle with the properties of an electron (one electron charge, one electron mass etc.). Electrons are extremely small and considered to be point particles, but their wave function can be dispersed over a large area (entire atom or even molecule).
A proton is not a fundamental particle since it is composed of three quarks (these quarks are fundamental).
All electrons are identical to each other. So if I have two electrons there is no experiment that I can perform to tell them apart. If there was they would behave drastically differently.
Electrons are fermions which means that no two electrons are allowed in the same place at the same time. If they were then there would be no chemistry since all electrons would drop into the lowest energy atomic shell.
Electrons also have a property called spin. It is LIKE the particle is spinning on its axis (despite being a point particle with no axis), it is only allowed to spin at one speed and in one of two directions.
but we weren't actually told what the particles were!
Well this is a deep philosophical question with no universally agreed answer.
Classically we imagine it is just a tiny charged billiard ball that bounces around, but this view is very wrong.
In quantum mechanics an electron is considered to be a wave function that interacts with other wave functions, all of which is treated very mathematically, with no classical analog.
In Quantum field theory and string theory the electron is considered an excitation of a string or field. Imagine a belt with a twist in it, the twist is the electron, it can move from one end to the other, it can be in a very spread out or exist in one spot, and if it meets a twist in the opposite direction then the both disappear. The twist in the opposite direction being an anti particle positron.
And if we don't know, how can we fire them from guns, like they do in experiments ... etc
If we don't understand something there is nothing stopping us from doing experiments and firing them from guns. In fact once we do understand something that's when we stop doing experiments.
If I don't know what a platypus is there is nothing stopping me firing it in a gun.