A consonant is a sound in spoken langage (or a letter of the alphabet denoting such a sound) that has no sounding voice (vocal sound) of its own, but must rely on a nearby vowel with which (con = Latin for "with") it can sound (sonant). A few sounds can function as either vowels or consonants, such as /l/, /r/, /m/, /n/ (though these are all consonants in English). The sounds /j/ as in English YOKE and /w/ as in English WOMAN are sometimes called semi-vowels, because although they function as consonants in some languages (e.g. English or Latin), phonetically they are vowels, or to be more exact, very short realizations of [i] and [u] respectively.
Consonants in the Latin alphabet are BCDFGHJKLMNPQRSTVWXZ. The letter Y is a consonant in "yoke" but a vowel in "myth". Originally, Y was a vowel letter in Greek, representing /u/ (later on, /y/ and in Modern Greek, /i/), and it normally has the sound value /y/ in German, in Finnish and other Scandinavian languages. The letter Y nicely shows how letters change their function. In Afrikaans, Y denotes the diphthong /EI/, probably as a result of mixing lower case i and y. In Dutch, Y appears only in loan words and is usually pronounced /i/. Italian, too, has Y only in very few loan words. Obviously, consonants and vowels are difficult to transcribe adequately with the alphabet we use in everday life. Therefore, linguists have devised phonetic alphabets such as the IPA alphabet or the computer readable SAMPA script.
Some common consonants arranged according to these properties (horizontal - manner; vertical - place), with voiceless and voiced consonants given in the form /t/-/d/, and rounded forms marked by an asterix:
Stops Fricatives Nasals Approximants Bilabial /p/-/b/ -/m/ Labiodental /f/-/v/ Dental /t/-/d/ /T/-/D/ -/n/ -/r/ Alveolar /t/-/d/ /s/-/z/ -/n/ -/r/ Postalveolar /t/-/d/ /S/-/Z/ -/n/ -/r/ Palatal -/j/ Velar /k/-/g/ -/N/ -/w/* Uvular Laryngeal /h/ Retroflex Lateral -/l/
Traditionally the above diagram is shown with place of articulation in the horizontal direction and manner of articulation in the vertical direction. (Perhaps somebody might redo this with a HTML-table)
For English speakers, /D/ is as in this, /j/ as in you, /N/ as in song, and /T/ as in thing. There are actually two sorts of /l/, the normal one as in liquid and a velarized form as in all. /t/ and /d/ are usually alveolar in Germanic languages but dental in Romance languages, and postalveolar in the affricates /tS/ and /dZ/.
Many languages also distinguish between aspirated and unaspirated stops, depending on whether there is a release of air after the noise. A stop can also release easily into a fricative, giving us what is called an affricate. Finally, voice refers to whether or not the vocal chords are moving, so we have both voiced and voiceless consonants.