Microchannels are structures of smaller hydraulic diameters used for efficient heat exchange in fluid flow. In this paper, a detailed study on types of channels categorized by different authors based on the size range has been done. These channels are further elaborated based on different geometries such as circular, rectangular and trapezoidal microchannels with hydraulic diameters range and aspect ratios. Major applications of microchannels can be seen in a diverse range of industries. Microchannels are best known for heat removal due to their compactness and high efficiency. A comprehensive study on the advantages of small hydraulic diameters has been done. Various correlations proposed by several authors to determine the heat transfer and friction factor for gas and liquid flow through a microchannel have been discussed. In microchannels, the Reynolds number range for different types of flow categorized as laminar, turbulent, and transition is different from the conventional range. The correlations suggested in the literature for heat transfer and friction factor are compared for the laminar and turbulent regime for liquid and gas flow of Prandtl number 4 and 0.7, respectively.