In human communication, voice is the preferred method. But in order to fit a band limited storage space or some transmission channel, speech signals have to be converted to formats using various techniques called as speech coding techniques. Today, speech coders have become an essential component in cellular communications, video conferencing, multimedia applications and computer based games. In the past decade, progress has been made towards the development of low-rate speech coders to be used in various applications which led to the development of new speech coders capable of producing high-quality speech reconstruction at low data rates. In order to optimize the performance, most of these coders employ the mechanisms to represent the spectral properties of speech, provided for speech waveform matching and hence achieve improved compression as compared to direct quantization. A number of these coders have already been adopted in national and international cellular telephony standards. This paper presents a review of various speech coders based on various parameters, their merits, demerits, types and applications.