An ionic substance will typically consist of a metal and a non-metal, such as AgCl.
Covalent molecules consist of non-metal atoms, such as CO2
The metallic substance in an ionic bond will donate it's electrons to the non-metal in order for them both to become more stable by having the same electron configuration as their neighboring noble gas. (Have 8 valence electrons.)
As the name suggests, covalent bonding involves the sharing of valence electrons to be stable for the same reason as above.