In an ionic bond, the atoms are bound together by the attraction between oppositely-charged ions. For example, sodium and chloride form an ionic bond, to make NaCl, or table salt.
In a covalent bond, the atoms are bound by shared electrons. If the electron is shared equally between the atoms forming a covalent bond, then the bond is said to be nonpolar.
The electronegativity difference between the bonded atom tells about the type of bond they are going to form.
One way to predict whether a bond is ionic or covalent is to look how far apart the two atoms forming the bonds are in the periodic table. If one atom is of the far left (Group 1 or 2) and the other is on the far right (Group 5, 6, or 7), then the atoms will have large differences in EN and will form an ionic bond. Most other pairs of atoms are close enough in EN to form covalent bonds. For example, any two atoms in the main group elements (Groups 3, 4, 5, 6, 7), will usually form a covalent bond. Remember to treat hydrogen like a Group 3 element as its EN is closest to boron.
KCl & NaCl are ionic and HCl & CCl4 are covalent.