Unmanned Aerial Vehicles (UAVs) have recently attracted attention in military areas as well as a wide range of commercial and civilian applications. With UAVs equipped with advanced transmitters and sensors, and with high mobility and flexibility in deployment, they have gained a special place in the field of information technology. Since there are several types of UAVs available, depending on circumstances choosing an appropriate one is essential for proper use of them. In this paper, we review some types of UAVs and a variety of UAV-enabled wireless networks with a focus on optimizing UAV position and flight paths. Since using UAVs is considered as one important complement for future cellular networks like 5G and Beyond 5G networks in disasters we focus on UAV placement optimization in cellular networks and then, we investigate the optimal UAV location for various scenarios and applications, and introduce methods of UAVs placement and trajectory design optimization.