The attraction between atoms is the result of a positive charge of one atom and the negative charge of another.
This is true for ionic where the transfer from electrons is from one atom to the other, and in covalent where an electron(s) is provided by both atoms. However these atoms are not evenly spaced out.
Some atoms can attract the electrons more easily than other, otherwise known as electronegativity
Electronegativity is the power of an atom to attract a lone pair of electrons in a covalent bond towards itself.
When considering the electrons as charge clouds, electron density is often used when describing the way, the negative charge is distributed in a molecule
The Noble gases are not included as they cannot form covalent bonds

General Trends

The Pauling scale is a measurement of electronegativity rangeing from 0 to 4 (4 being the most electronegative)
Electronegativity increases along a period as the nuclear charge increase whilst the number of inner shells remains the same
Electronegativity decreases down a group. This is due to the increasing size of the atomic radii down a group, meaning there is less attraction to the outer electrons and more electron shielding
The most electronegative atoms are found in the top right

Electronegativity Requirements

Electronegativity Depends on:
Nuclear Charge
Distance between the nucleus and the outer electrons
Electron shielding

The smaller the atom, the closer the nucleus is to the shared outer main level of electrons and therefore the greater electronegativity
The larger the nuclear charge (for a given shielding effect) the greater the electronegativity