What is the difference between covalent substances and molecular substances?

The terms covalent and molecular are often used interchangeably to describe the same type of substance. However, there is a distinction between the two. The term covalent refers to a type of bonding in which pairs of valence electrons are shared by two atoms. Covalent compounds are those that exhibit covalent bonding. 

Molecular compounds are a type of covalent compound. Molecular compounds exist as individual molecules.They have tend to have low melting and boiling...

The terms covalent and molecular are often used interchangeably to describe the same type of substance. However, there is a distinction between the two. The term covalent refers to a type of bonding in which pairs of valence electrons are shared by two atoms. Covalent compounds are those that exhibit covalent bonding. 

Molecular compounds are a type of covalent compound. Molecular compounds exist as individual molecules. They have tend to have low melting and boiling points because phase changes involve overcoming intermolecular attractions but not breaking covalent bonds.

Here are some examples:

Methane, `CH_4` , is a gas consisting of individual molecules at room temperature.

Ethanol, `C_2H_5OH` , is a liquid consisting of individual molecules at room temperature. When heated the molecules move farther apart and eventually enter the gas phase, but the covalent bonds remain intact.

Paraffin, a wax with the formula `C_31H_64` , is a molecular compound with a melting point of 37ºC. It's a solid at room temperature due to intermolecular forces. When heated the molecules enter the liquid phase but bonds within the molecules aren't broken.

Most covalently bonded substances are molecular.

Covalent network solids are a second type of covalent compound. They're not truly molecular because they exist as a crystal lattice held together by covalent bonds. Their structure is similar to that of an ionic solid in that there are repeating formula units rather than separate molecules of a specific formula. Covalent network solids have very high melting points because melting involves breaking covalent bonds. Examples are quartz and diamond.

In summary, all molecular substances are covalently bonded but not all covalently bonded substances are molecular.