, a velar
sound is made with the back of the tongue
on or near the velum
or soft palate
, in the back of the mouth.
The most common velar sounds are the plosives k (as in (kit, cat, queen) and g (as in get, got). I don't know of a single language in the world that doesn't have a k-sound, though apparently there are some.
The velar fricatives don't occur in standard English but the voiceless one (phonetic symbol [x]) is common enough in the world's languages: loch, Achtung, chutzpah, Khalid. It is the sound in Chinese written h in pinyin. In the speech of some Londoners it is used instead of k.
The corresponding voiced one is much rarer: the Arabic gh. The g of Spanish when between vowels is a weaker (approximant) form of this.
The velar nasal is the sound represented by English ng as in sing, singer. In English this can occur in the middle or at the end of a word, but never at the beginning. But many languages do allow it at the beginning, e.g. Swahili ng'ombe 'cow', Samoan galue 'work'.
In English it also occurs before the velar plosives, as in ink, finger. In some languages (e.g. Italian, Spanish, Hungarian) it cannot occur by itself at all, but is used in these groups, e.g. cinque, cinco. Historically this was also true for English: a word like sing ended in a real g sound, which caused the preceding n to be velar; then the final g was lost in pronunciation.
The velum is involved in phonetic effects in several ways, the rest of which deserve their own nodes, but briefly:
What's described above is velar closure in the case of k, g, where the tongue touches the velum.
Entirely different is velic closure, which is when the velum is up, blocking air from going into the nasal cavity. When the velum is down, nasal sounds are created by the resonance through the nose: the consonants like n, m, ng and the vowels of French un bon vin blanc.
When velic closure is used to create pressure differences in the mouth before release, the result is the velaric air stream mechanism that creates the Southern African sounds known as clicks.
A tongue that's mainly occupied in the front of the mouth can simultaneously have the back raised towards the velum. This gives the sound a "darker" colouring. This is the difference in English between the L sounds of law and all: at the end of a syllable an English L is velarized. In Russian, most consonants can be either velarized or palatalized.