Electronegativity: The ability of an atom to attract a bonding pair of electrons

In general, electronegativity

Decreases down a group
Increases across a period

If two atoms of the same element are bonded together by overlap of atomic orbitals, the electrons will be evenly distributed between the two nuclei, since both atoms have equal abilities to attract electrons.

However, if two atoms of different elements are bonded together, they will have different abilities to attract electrons. This leads to an uneven distribution of electrons between the two nuclei. The atom with higher electronegativity will have more electron density.

The atom with more electron density will have a slightly negative charge - represented by \(\delta -\) and the the atom with less electron density will have a slightly positive charge - represented by \(\delta +\). The overall molecule is still neutral.

This type of bond is known as a polar covalent bond. It can be thought of as a covalent bond with some ionic character.