<p>Speech is a basic form of communication for conveying messages amongst humans and machines, and human-machine interface. But it is susceptible to interferences from background noise or environmental noise that can distort the signal and reduce intelligibility of the original message. Therefore, speech enhancement is a field of significant importance and wide applications, especially in the noisy environment or natural background settings. In this tutorial paper, speech enhancement techniques using different signal processing methods are reviewed, and related information is provided in a comprehensive way, to help the researchers. The efficacy of speech enhancement techniques is evaluated mostly by separating the speech signal from noise. Hence, different speech signal and noise mixing techniques are reviewed, apart from enumerating different speech databases and noise sources. Different signal processing methods used for speech enhancement are reviewed, along with performance evaluation measures. The need for speech enhancement is also discussed with different application scenarios. In addition, a comparison of different features, signal processing methods used for extracting those features, and metrics for their performance measure in speech enhancement, is presented. This paper should be quite informative and helpful to the researchers, those working in the domains of speech enhancement or extracting speech signal from noisy background or real-life natural application scenarios.</p>