Ionic bonding occurs when transfer of electrons takes place. One atom (or molecule) donates one or more electrons to another. Since electrons have a negative charge, this leaves the donating atom electron deficient (+ve charged ion) and the other atom electron rich ( -ve charged ion). The ions then attract each other throughelectrostatic forces of attraction as they are oppositely charged.
Covalent bonding occurs when atoms/molecules share pairs of electrons.
Metallic bonding is bonding that occurs in metals. This leads to giant structures of metal atoms arranged in a regular pattern. The outer shell electrons of metals are delocalised (free to move around) and so a metallic structure is a regular arrangment of +ve charged ions with negative electrons in between, held together by electrostatic forces of interaction.