Organic compounds are classified into two main categories based on the presence or absence of a closed-loop of atoms in their molecular structure: acyclic and cyclic compounds.

Acyclic Compounds:
Acyclic compounds, also known as aliphatic compounds, are organic compounds that do not contain any closed-loop of atoms in their molecular structure. They are also called open-chain compounds because their structure resembles an open chain or a straight line. Acyclic compounds can be either saturated or unsaturated. Saturated acyclic compounds contain only single bonds, while unsaturated acyclic compounds contain one or more double or triple bonds. Examples of acyclic compounds include alkanes, alkenes, and alkynes.

Cyclic Compounds:
Cyclic compounds, also known as closed-chain compounds or ring compounds, are organic compounds that contain one or more closed-loop of atoms in their molecular structure. They are formed by the bonding of the ends of a chain of atoms to form a ring. Cyclic compounds can be either aromatic or non-aromatic. Aromatic compounds contain a ring of atoms with alternating double bonds, while non-aromatic compounds have rings with other types of bonding patterns. Examples of cyclic compounds include cycloalkanes, cycloalkenes, and benzene.

In summary, acyclic compounds do not contain any closed-loop of atoms in their molecular structure, while cyclic compounds contain one or more closed-loop of atoms in their molecular structure.