A consonant is a sound that is made by either constricting or completely closing the vocal tract. Consonants differ in their place of articulation and in their manner of articulation. The IPA Consonant chart is a visual organization of consonants in the world's natural languages.
Consonants may also be categorized by other important features, mainly voicing.
Manner of Articulation
Plosives (or stops) are made by closing the vocal tract for a short time and then opening it again so that a sound is heard.
Many languages (but far from all) make a contrast between voiced (vocal cords vibrating) and unvoiced (vocal cords not vibrating) plosives. Some languages make a contrast between aspirated and unaspirated stops. Aspirated stops are followed by a noticeable burst of air (/h/-like), while unaspirated stops are not. In English and many other languages, aspirated and unaspirated stops are not contrastive phonemes, but rather an allophonic variation.
Some languages, such as Chinese or Icelandic, don't make a contrast between voiced and unvoiced stops, but between aspirated and unaspirated. Other languages, such as Thai, have a three-way contrast between aspirated, unaspirated ("plain") and voiced stops. There are also languages (such as Hindi) with a four-way contrast, between unvoiced unaspirates, unvoiced aspirates, voiced aspirates, and voiced unaspirates.
Some languages have contrastive types of plosives that are unfamiliar to many speakers of European languages, such as various forms of glottalized stops.
Voiced plosives include /b/, /g/, and /d/. Unvoiced plosives include /p/, /k/, and /t/.
Fricatives are made by contracting the vocal tract, so that a turbulent sound is created. Many languages make a contrast between voiced and unvoiced fricatives in the same way that plosives do.
Voiced fricatives include /v/, /z/, and /ʒ/ (zh/ge). Unvoiced fricatives include /f/, /s/, and /ʃ/ (sh).
Nasals are made by completely sealing off the vocal tract, forcing air out the nose. This results in a vibrating sound. Most languages have only voiced nasals. There are some examples of languages with unvoiced nasals, such as Icelandic.
Nasals include /n/ and /m/.
There is also a velar nasal /ŋ/ that occurs in many languages. In English it is often written with the digraph ng. The velar nasal cannot begin a word in English, unlike in many other languages.
Place of Articulation
Labial consonants are made by constricting the vocal tract at the lips, either with both lips (bilabials) or with the lower lips and the teeth (labiodentals). /m/, /p/, /b/, /f/ and /v/ are labials.
Coronal consonants are made by constricting the vocal tract between the tip or front part of the tongue and the teeth (dentals - such as English 'th' sounds), right behind the teeth on the alveolar ridge (alveolars), touching the back of the ridge (postalveolars - 'ch' and 'sh'), or just behind the ridge (retroflexes - similar to English 'r'). /t/, /d/, /n/, /s/ and /z/ are coronal consonants, alveolar in particular.
Dorsal consonants are made by placing the tongue behind the alveolar ridge on the hard palate (middle part of the mouth's roof) (palatals), on the soft palate (back of roof) (velars), or at the uvula (uvulars). /k/, /g/, /x/, /ɣ/, and /c/ are dorsal consonants.
Radical consonants are made by placing the root of the tongue (back of the tongue) against the pharynx (pharyngeal) or with the aryepiglottic folds against the epiglottis (epiglottal). /ʕ/, /ʜ/ and /ʡ/ are radical consonants.
Glottal consonants are made by the glottis. /ʔ/ and /h/ are glottal consonants.
There are some consonants that have two places of articulation, such as /w/, which is both velar and bilabial. Affricates are plosives that transform into fricatives after the constriction is released.
Clicks, implosives, and ejectives are other types of consonants that aren't made by blowing air through a place of articulation.
Natural languages follow several patterns in consonant systems which are important to consider when trying to create a naturalistic conlang.
Voiceless stops, such as /p/, /t/, and /k/, are by far the most common sounds. Nasals, especially /n/ and /m/, are also very common. However, some natural languages lack a few of these sounds.
/s/ is the most common fricative; the glides /j/ and /w/ are also incredibly common. Many languages have /l/ and/or some rhotic consonant as well.
After these basic consonants, the directions in which a language expands vary; more placements of articulation (adding uvular /q/, the palatal affricate /tʃ/, or a glottal stop), more fricatives (/h/ and /ʃ/ are common), or phonemic voicing (most likely of just the stops - /b/ and /p/).
Sounds that are more rare in the world's languages are voiced dorsals (/g/, /ɣ/, etc), uvular and radical sounds in general (except /h/ and glottal stop), voiced fricatives (/v/, /z/), dental fricatives (/θ/, /ð/), retroflex sounds, and palatal stops.
Some consonants, while not particularly uncommon on their own, are unlikely to occur together in a language; for instance, while bilabial and labiodental fricatives (ɸ β vs f v) are both fairly common, only a few languages have fricatives in both places, since they are similar sounds that are easy to confuse.