Phonetics is a discipline of linguistics that focuses on the study of the sounds used in speech. It is not concerned with the meaning of these sounds, the order in which they are placed, or any other factor outside of how they are produced and heard, and their various properties. This discipline is closely related to phonology, which focuses on how sounds are understood in a given language, and semiotics, which looks at symbols themselves.
There are three major subfields of phonetics, each of which focuses on a particular aspect of the sounds used in speech and communication. Auditory phonetics looks at how people perceive the sounds they hear, acoustic phonetics looks at the waves involved in speech sounds and how they are interpreted by the human ear, and articulatory phonetics looks at how sounds are produced by the human vocal apparatus. This third subfield is where the majority of people begin their study, and it has uses for many people outside of the field of linguistics. These include speech therapists, computer speech synthesizers, and people who are simply interested in learning how they make the sounds they do.
The International Phonetic Association has a special alphabet for describing all of the different sounds, or phones, currently thought to be used in human speech. The International Phonetic Alphabet (IPA) has more than 100 distinct phones listed and given distinct notation. Sounds can be separated into a number of different groups, based on whether they use air from the lungs or not, whether they are voiced or not, the position of the tongue in the mouth, and how the sound is altered. While the bulk of sounds made by the speakers of the world fall into a somewhat narrow band of this spectrum, there are other sounds that are quite different, such as the clicks and smacking sounds made in some African languages.
Most consonants, called pulmonic consonants, use air from the lungs and can be placed on a grid depending on which parts of the vocal tract are used to articulate the speech sound and how air is obstructed as it passes through the mouth. For example, the sound /p/ uses both lips to articulate air, and is therefore known as a bilabial. It also consists of a full stop of air, known as a plosive. The /p/ sound, therefore, as well as the /b/ sound, can be described as a bilabial plosive. The /b/ sound, since the vocal fold is vibrating as it is said, is called a voiced bilabial plosive, while the /p/ sound, which has no such vibration, is called an unvoiced bilabial plosive.
All the consonant sounds used in speech can be described in this manner, from the /r/ sound in English, which is called an alveolar trill, for example, to the sound at the beginning of the word "yet," transcribed in IPA with the symbol j and described as a palatal approximant, to the deep-throated Arabic sounds of the pharyngeal fricatives.