Ionic compounds are composed of positive and negative ions (atoms with extra or fewer electrons than their neutral counterparts). In such compounds the total charge of the positive and negative ions always adds up to zero. The attractions of the opposite charges holds the compounds together.

Molecular compounds are composed of neutral atoms which are held together by sharing some of their electrons in common. No separate charges are involved.

How can you distinguish one kind of compound from the other?

In general ionic compounds are composed of metals and non-metals. Molecular compounds typically consist of only non-metals. In water solutions ionic compounds are electrolytes. Molecular compounds generally are not.