Microelectromechanical systems (MEMS) technology has provided the platform for the miniaturization of analytical devices for biological applications. Beside the fabrication technology, the study and understanding of flow characteristics of fluid in micrometer or even nanometer scale is vital for the successful implementation of such miniaturized systems. Microfluidics is currently under the spotlight for medical diagnostics and many other bio-analysis as its physical size manifested numerous advantages over lab-based devices. In this review, elementary concepts of fluid and its flow characteristics together with various transport processes and microchannel condition are presented. They are among the fundamental building block for the success in microfluidic systems. Selected application examples include biological cell handling employing different schemes of manipulation and DNA amplification using different microreactor arrangement and fluid flow regime.