The field of multiferroics has expanded significantly in recent years, with many new materials discovered. This review classifies these materials based on their microscopic origins and explores how similar multiferroic behavior might be found in existing systems. Multiferroics combine ferroelectric and magnetic properties, allowing for magnetic responses to electric fields and vice versa. This property enables new applications such as 4-state logic and magnetoelectric sensors. Multiferroics are divided into two main types: Type-I and Type-II. Type-I multiferroics have separate sources for ferroelectricity and magnetism, with ferroelectricity often occurring at higher temperatures. Examples include BiFeO3 and YMnO3. Type-II multiferroics, discovered more recently, have magnetism causing ferroelectricity, indicating strong coupling. These materials typically have smaller polarization. The microscopic mechanisms behind multiferroicity include lone pairs, charge ordering, and geometric effects. For example, in BiFeO3, lone pairs of Bi³+ and Pb²+ contribute to polarization. Charge ordering can also lead to ferroelectricity, as seen in materials like Pr1/2Ca1/2MnO3. Geometric effects, such as the tilting of MnO5 blocks in YMnO3, can also generate polarization. Type-II multiferroics often involve spiral magnetic structures, such as cycloidal spirals, which can lead to polarization. Theoretical models, like the one involving spin-orbit interaction, explain the polarization in these materials. Magnetic frustration is a key factor in spiral magnetic ordering, leading to new findings with potential applications. Type-II multiferroics with collinear magnetic structures can also exhibit ferroelectricity due to exchange striction. For example, in Ca3CoMnO6, magnetic ordering breaks inversion symmetry, leading to ferroelectricity. Additionally, frustrated magnets can exhibit electronic ferroelectricity, where polarization is proportional to spin correlation functions. The study of multiferroics has led to new insights and applications, such as controlling domain walls with electric fields and creating electromagnons. These materials also have potential in magnetic memory and sensors. The field is highly active, with ongoing research into new materials and applications. Future directions include the development of "artificial" composite multiferroics and the investigation of dynamical properties and excitations in these materials. The field of multiferroics is expected to continue growing, with significant potential for both fundamental research and practical applications.The field of multiferroics has expanded significantly in recent years, with many new materials discovered. This review classifies these materials based on their microscopic origins and explores how similar multiferroic behavior might be found in existing systems. Multiferroics combine ferroelectric and magnetic properties, allowing for magnetic responses to electric fields and vice versa. This property enables new applications such as 4-state logic and magnetoelectric sensors. Multiferroics are divided into two main types: Type-I and Type-II. Type-I multiferroics have separate sources for ferroelectricity and magnetism, with ferroelectricity often occurring at higher temperatures. Examples include BiFeO3 and YMnO3. Type-II multiferroics, discovered more recently, have magnetism causing ferroelectricity, indicating strong coupling. These materials typically have smaller polarization. The microscopic mechanisms behind multiferroicity include lone pairs, charge ordering, and geometric effects. For example, in BiFeO3, lone pairs of Bi³+ and Pb²+ contribute to polarization. Charge ordering can also lead to ferroelectricity, as seen in materials like Pr1/2Ca1/2MnO3. Geometric effects, such as the tilting of MnO5 blocks in YMnO3, can also generate polarization. Type-II multiferroics often involve spiral magnetic structures, such as cycloidal spirals, which can lead to polarization. Theoretical models, like the one involving spin-orbit interaction, explain the polarization in these materials. Magnetic frustration is a key factor in spiral magnetic ordering, leading to new findings with potential applications. Type-II multiferroics with collinear magnetic structures can also exhibit ferroelectricity due to exchange striction. For example, in Ca3CoMnO6, magnetic ordering breaks inversion symmetry, leading to ferroelectricity. Additionally, frustrated magnets can exhibit electronic ferroelectricity, where polarization is proportional to spin correlation functions. The study of multiferroics has led to new insights and applications, such as controlling domain walls with electric fields and creating electromagnons. These materials also have potential in magnetic memory and sensors. The field is highly active, with ongoing research into new materials and applications. Future directions include the development of "artificial" composite multiferroics and the investigation of dynamical properties and excitations in these materials. The field of multiferroics is expected to continue growing, with significant potential for both fundamental research and practical applications.