Intelligent Reflecting Surface (IRS) is a novel technology that enhances wireless communication by smartly reconfiguring the propagation environment using low-cost passive elements. This paper provides an overview of IRS, its applications, advantages, and challenges in designing IRS-aided hybrid wireless networks. IRS can achieve fine-grained 3D beamforming, enabling signal enhancement or interference cancellation. It is particularly useful in scenarios such as coverage extension in mmWave communications, improving physical layer security, suppressing interference, enabling D2D communications, and supporting simultaneous wireless information and power transfer (SWIPT). IRS is a planar surface with many passive reflecting elements that can independently adjust the amplitude and phase of incident signals. Unlike traditional active relays, IRS does not require active transmit modules and operates in full-duplex mode, making it more spectrally efficient. It is also more cost-effective and energy-efficient than active surface-based massive MIMO. IRS can be deployed in existing wireless systems without requiring changes to hardware or protocols, offering flexibility and compatibility. The hardware architecture of IRS is based on metasurfaces, which are digitally controllable 2D metamaterials. Each element can be controlled via electronic devices such as PIN diodes or MEMS switches to adjust reflection amplitude and phase. However, practical implementations often use discrete amplitude and phase-shift levels due to cost and complexity constraints. Key challenges in designing IRS-aided networks include passive beamforming design, channel acquisition, and deployment. Passive beamforming requires optimizing discrete amplitude and phase-shift values, which is computationally intensive. Channel acquisition involves estimating IRS-BS/user channels, which can be done using conventional methods or feedback from BS/users. Deployment requires careful placement to optimize coverage and minimize interference. Numerical results show that IRS can significantly enhance signal power and suppress interference. For example, the BS transmit power required to achieve a target SNR decreases with more IRS elements, demonstrating the effectiveness of IRS in creating "signal hotspots" and "interference-free zones." IRS also improves physical layer security by canceling eavesdropping signals. The integration of IRS into future wireless networks is expected to change the architecture from traditional active-only systems to hybrid systems with both active and passive components. This opens new research directions in optimizing network capacity and performance.Intelligent Reflecting Surface (IRS) is a novel technology that enhances wireless communication by smartly reconfiguring the propagation environment using low-cost passive elements. This paper provides an overview of IRS, its applications, advantages, and challenges in designing IRS-aided hybrid wireless networks. IRS can achieve fine-grained 3D beamforming, enabling signal enhancement or interference cancellation. It is particularly useful in scenarios such as coverage extension in mmWave communications, improving physical layer security, suppressing interference, enabling D2D communications, and supporting simultaneous wireless information and power transfer (SWIPT). IRS is a planar surface with many passive reflecting elements that can independently adjust the amplitude and phase of incident signals. Unlike traditional active relays, IRS does not require active transmit modules and operates in full-duplex mode, making it more spectrally efficient. It is also more cost-effective and energy-efficient than active surface-based massive MIMO. IRS can be deployed in existing wireless systems without requiring changes to hardware or protocols, offering flexibility and compatibility. The hardware architecture of IRS is based on metasurfaces, which are digitally controllable 2D metamaterials. Each element can be controlled via electronic devices such as PIN diodes or MEMS switches to adjust reflection amplitude and phase. However, practical implementations often use discrete amplitude and phase-shift levels due to cost and complexity constraints. Key challenges in designing IRS-aided networks include passive beamforming design, channel acquisition, and deployment. Passive beamforming requires optimizing discrete amplitude and phase-shift values, which is computationally intensive. Channel acquisition involves estimating IRS-BS/user channels, which can be done using conventional methods or feedback from BS/users. Deployment requires careful placement to optimize coverage and minimize interference. Numerical results show that IRS can significantly enhance signal power and suppress interference. For example, the BS transmit power required to achieve a target SNR decreases with more IRS elements, demonstrating the effectiveness of IRS in creating "signal hotspots" and "interference-free zones." IRS also improves physical layer security by canceling eavesdropping signals. The integration of IRS into future wireless networks is expected to change the architecture from traditional active-only systems to hybrid systems with both active and passive components. This opens new research directions in optimizing network capacity and performance.