what is covalent bond?

Dear Student,
Covalent bond is the bond formed between two atoms when they share their outermost electrons, in order to achieve an inert gas configuration. We know that an ionic bond is formed between a metal atom (an electropositive atom) and a non-metal atom (an electronegative element) by transfer of electrons. A covalent bond is formed between atoms both of which are non-metals. Both do not have the tendency to lose electrons but to gain electrons, hence they share their valence electrons and complete their octets. This sharing of electrons results in bond formation between the two atoms. 

For example, consider carbon tetrachloride. Carbon has 4 electrons in its valence shell and needs 4 more in order to complete its octet. Chlorine, on the other hand has 7 electrons in its valence shell and needs one more in order to complete its octet. Therefore there is the formation of covalent bond in carbon tetrachloride, C shares the four electrons with 4 Cl atoms, resulting in the formation of 4 covalent bonds. 
Regards