Over the past seventeen years, notable advances have occurred in a diverse range of scientific areas following the development of femtosecond optical frequency combs. Frequency comb first establishes a direct link between microwave and optical frequencies, thus enabling the breakthrough of time and frequencydomain research. Frequency stabilization of comb has led to a revolution in frequency metrology and precision measurement. In addition, welldefined temporal coherence across the optical spectrum makes it a unique tool for molecular spectroscopic applications, simultaneously providing high speed measurement, high spectral resolution and broad spectral coverage. This tutorial review provides an introduction to femtosecond optical frequency combs, covering their principles of operation and applications and advance in frequency metrology, absolute distance measurement, precision absorption spectroscopy, high speed nonlinear spectroscopy and microscopy and high precision time and frequency transfer. In this way it aims to demonstrate their potential as a spectroscopic tool that could play a very significant role in future advances in the precision sciences.