Nutshell phonetics of nasals
Nasals are speech sounds which are produced by means of a constriction which completely blocks the airflow through the mouth, however, the air is allowed to flow through the nose. In [n] for example, the tip of the tongue touches the alveolar ridge and both sides of the tongue make contact with the palate too. This creates an obstruction which prevents the air to flow out of the mouth. However, in nasals the velum is lowered so that the air is directed into the nasal cavity through which it can escape freely.
The vast majority of languages in the world have voiced nasals. Voiceless nasals are very rare. Examples of languages with voiceless nasals are Burmese, Welsh and Icelandic. Nasals can also be modified by different voice qualities such as breathiness (Tsonga, southern Africa) or creaky voice (in north-west American Indian languages such as Kwakw'ala).
For the phonetic transcription of nasals, the phonetic notation system of the International Phonetic Association distinguishes between 7 discrete phonetic symbols. The symbols for other nasal sounds are derived from these by adding diacritics. Most phonetic symbols and some of the diacritics are illustrated in the videos above which you may find useful as ear training exercises in practical phonetics. In a practical phonetics perspective, the nasals are characterised by a low frequency resonance and they may not always be easily identified: watching the speaker is equally important as listening. In order to avoid clutter and competition with other diacritics, I have consistently placed the voiceless diacritic above the main symbol for the nasal.
In terms of their place of articulation, the most common nasals in languages are the labial, alveolar and velar which very often co-occur in sound systems. Some languages have nasals at 6 places of articulation such as Malayalam (Pakistan): this language has a labial, dental, alveolar, retroflex, palatal and a velar nasal. There are very few languages which have no nasals at all, but an example of such a language is the American Indian Language Quileute.
The most frequent nasals in languages of the world are:
[ m ] > [ n ] > [ ŋ ] > [ ɲ ] > [ n̪ ]
In some nasals there is a stricture of complete closure at two different places in the mouth: these are double articulations. The labial-velar is particularly frequent. Although this sound is referred to as a "labial-velar" nasal, it is transcribed by a combination of the phonetic symbol for the velar nasal followed by the symbol for the labial nasal. Labial-velar nasals are particularly common in West and Central African languages.
The videos below illustrate the processes involved in the production of nasals and they appear in languages like Dutch and English. In all cases you should note the full obstruction in the mouth which is created by contact between the lips or contact between the tongue and the roof of the mouth. The velum is not raised so that the airflow is directed into the nasal cavity. In all the examples below, the vocal folds vibrate.
Visual illustration of a few nasal articulations