Electricity is the flow of positive or negative charges in response to electric forces (an electric field). Static means something is not moving, but there is still an electric field.
What is happening is normally positive and negative charges occur in equal amounts everywhere, so there is no net charge. We say they cancel each other out. If you have more of one or the other, then there is a net charge (an imbalance, as your text states). Since opposite charges attract, another charge would feel a force towards this imbalance. Similarly, the imbalanced charge would feel a force due to other charges surrounding it (ones that aren't cancelled out). If you live in a dry climate you may know the sensation of your hair standing on end after rubbing a balloon on it. What happens is each hair has some extra charge deposited on it, and all these charges are repelling eachother, taking your hair with it.
We say it's static because the charges can't go anywhere. This is because the extra charge displaces the charges nearby it, pulling the opposite ones closer and pushing the same ones farther away. What's important is that it can't push these other charges very far (the material is not a conductor). We say it "polarizes" the material.
Even though the net charge of one of your strands of hair might be negative, and be repelled by other strands, the extra electrons have surrounded themselves with positive charges and are "stuck" in a sense. This is why you need friction to cause an imbalance of charges. You are overcoming this stickiness.
Now if you brought a conductor (e.g., metal) to the material, the charge can attach itself to the metal in the same way, (polarizing the material and then pulling itself towards opposite charges) but because charges on conductors can move around, the extra charge can flow more freely. The charge is still "stuck" to the conductor, but can move around inside the conductor.
Taking copper (atomic number 29) as an example of a good electrical conductor in the solid (and liquid) state.
On average there are positive copper ions Cu$^+$ with 28 orbiting (bound) electrons and for each ion one free/mobile (unbound) electron.
When no external electric field is applied these free electrons having thermal energy move about at random throughout the metal just like the molecules in a gas.
In the solid the positive copper ions are fixed in a lattice and vibrate about a mean position.
When an electric field is applied the free electrons can move under the influence of the electric field. It is these free electrons which result in copper being a good conductor of electricity (and heat). The positive ions are fixed into the lattice and so can only move their mean position very little.
If the final state is such that the net movement of the free electrons is zero (electrostatics) then the redistribution of charges throughout the metal is such that the electric field produces by the induced charges as a result of the movement of the free electrons is equal and opposite to the externally applied electric field and so the net field in the conductor is zero.
So when you see diagrams of conductor with positive and negative charges on them, the region labelled as having a negative charge is one where there is a net surplus of free electrons and the region and the region with positive charges is one which has a net deficit of free elctrons.
Best Answer
You observe that protons and electrons have opposite charges. It is a matter of definition which is positive, and the convention was made before anybody knew about protons and electrons-I have heard it was on the basis of a static charge.
It is the cathode that is negative relative to the anode. For conductors, the conduction electrons are free to move and not tied to a particular atom. But it is an excess of electrons on the cathode that makes it negative and a deficit of electrons on the anode that makes it positive.