Fe-based superconductors, such as LaFeAsO, have critical temperatures up to 55K, making them important for high-temperature superconductivity research. These materials exhibit diverse superconducting gap structures, which vary between families and even within families depending on doping or pressure. The apparent non-universality of these gaps can be explained by spin fluctuation theory and the electronic structure of these systems, combined with the likely "sign-changing s-wave" (s±) symmetry. The article reviews theoretical aspects, materials properties, and experimental evidence related to this hypothesis, and discusses further measurements needed to resolve these issues. Fe-based superconductors differ from cuprates and MgB2 in their electronic structure and superconducting properties. Cuprates have d-wave symmetry, while Fe-based superconductors may have different gap symmetries. MgB2 is a multiband superconductor with two distinct gaps, and Fe-based superconductors also exhibit multiband behavior. The superconducting state in cuprates is universally d-wave, but Fe-based superconductors may have different gap structures. However, the ultimate pairing mechanism in both systems may be fundamentally similar, though details like gap symmetry and structure depend on Fermi surface geometry, orbital character, and correlation levels. The electronic structure of Fe-based superconductors is complex, with multiple bands contributing to the Fermi surface. DFT calculations have been used to study this structure, but they have limitations, particularly in capturing strong correlations and long-range fluctuations. Minimal band models, such as those including d-orbitals, have been proposed to describe the electronic structure and superconducting properties of Fe-based superconductors. Spin fluctuation theories are used to explain the pairing mechanism in Fe-based superconductors. These theories suggest that spin fluctuations can lead to superconductivity, with the symmetry of the order parameter depending on the electronic structure and interactions. The symmetry of the superconducting gap can be s-wave, d-wave, or other forms, and the presence of nodes or full gaps depends on the material. Experimental evidence, such as ARPES and NMR measurements, has been used to study the gap structure and symmetry of Fe-based superconductors. Theoretical models and experimental studies have shown that Fe-based superconductors exhibit a range of superconducting properties, including multiband behavior, different gap symmetries, and the influence of disorder and magnetic impurities. The understanding of these properties is crucial for developing new superconducting materials and improving our knowledge of high-temperature superconductivity.Fe-based superconductors, such as LaFeAsO, have critical temperatures up to 55K, making them important for high-temperature superconductivity research. These materials exhibit diverse superconducting gap structures, which vary between families and even within families depending on doping or pressure. The apparent non-universality of these gaps can be explained by spin fluctuation theory and the electronic structure of these systems, combined with the likely "sign-changing s-wave" (s±) symmetry. The article reviews theoretical aspects, materials properties, and experimental evidence related to this hypothesis, and discusses further measurements needed to resolve these issues. Fe-based superconductors differ from cuprates and MgB2 in their electronic structure and superconducting properties. Cuprates have d-wave symmetry, while Fe-based superconductors may have different gap symmetries. MgB2 is a multiband superconductor with two distinct gaps, and Fe-based superconductors also exhibit multiband behavior. The superconducting state in cuprates is universally d-wave, but Fe-based superconductors may have different gap structures. However, the ultimate pairing mechanism in both systems may be fundamentally similar, though details like gap symmetry and structure depend on Fermi surface geometry, orbital character, and correlation levels. The electronic structure of Fe-based superconductors is complex, with multiple bands contributing to the Fermi surface. DFT calculations have been used to study this structure, but they have limitations, particularly in capturing strong correlations and long-range fluctuations. Minimal band models, such as those including d-orbitals, have been proposed to describe the electronic structure and superconducting properties of Fe-based superconductors. Spin fluctuation theories are used to explain the pairing mechanism in Fe-based superconductors. These theories suggest that spin fluctuations can lead to superconductivity, with the symmetry of the order parameter depending on the electronic structure and interactions. The symmetry of the superconducting gap can be s-wave, d-wave, or other forms, and the presence of nodes or full gaps depends on the material. Experimental evidence, such as ARPES and NMR measurements, has been used to study the gap structure and symmetry of Fe-based superconductors. Theoretical models and experimental studies have shown that Fe-based superconductors exhibit a range of superconducting properties, including multiband behavior, different gap symmetries, and the influence of disorder and magnetic impurities. The understanding of these properties is crucial for developing new superconducting materials and improving our knowledge of high-temperature superconductivity.