Uvular consonants are articulated with the back of the tongue against or near the uvula, that is further back in the mouth than velar consonants are. Most uvular consonants are either stops or fricatives, but a very small number of languages use them as nasals, trills, or approximants.
Uvular consonants are found in many African and Middle-Eastern languages, most notably Arabic, and Native American languages, as well as the letter "r" in French and German.
The unvoiced uvular stop is expressed as "q" in most transliteration schemes, and is pronounced like a "k" with the middle of one's tongue against the soft uvula rather than the velum. The most familiar use will doubtless be in the transliteration of Arabic place names to English (such as Qatar and Iraq), though most English speakers pronounce the sound as their nearest equivalent, "k".
The voiced equivalent of "q" is much rarer, and is written in SAMPA as /G\\/. It sounds like a "g" articulated in the same position as "q". There is no widely-used language that uses it, except some varieties of Persian.
The unvoiced uvular fricative is also exceedingly rare. It sounds similar to the "kh" (represented in IPA as "x") in Spanish, German, Russian, or Arabic, except that it is articulated on the uvula. It can be heard in French at the end of a word following t, c, or p, as in maître: the R is here a voiceless uvular.
The voiced uvular fricative is much more common in Europe: it is found in French as the usual value of the letter R, and has spread into some neighbouring languages. In German it is an approximant.