The word guttural literally means 'of the throat', and is derived from the Latin word for throat. In colloquial usage, the term is used for any sound pronounced in the throat or near the back of the mouth that is considered "harsh". The OED says,
Phonologists such as Miller (2005) and Pullum & Ladusaw restrict the term guttural to sounds articulated in the throat, which include pharyngeal, epiglottal, and glottal consonants (see radical consonant), and murmured, pharyngealized, and glottalized vowels (see strident vowel). The Tuu and Juu (Khoisan) languages of southern Africa have large numbers of guttural vowels. These sounds share certain phonological behaviors which warrant the use of a term specifically for them.
Historically Hebrew also had guttural consonants, but except for the guttural R, these were not found in most European dialects of Hebrew. In modern Israel, the guttural pronunciations of Mizrahi Jews were stigmatized by the Ashkenazi cultural elite for decades. Today, the most common pronunciation of Israeli Hebrew has no guttural consonants except for variants of /r/ and sometimes /χ/ or /ħ/, but traditional Mizrahi pronunciations, which include the full historical range of guttural consonants, are still used in music and poetry.
A guttural style of singing is very popular within extreme metal (Black, Death, Doom & sometimes Thrash Metal) music, as its aggressive sound arguably complements the music.
In the popular consciousness, some languages are considered to be guttural languages, as opposed to just possessing some sounds which are pronounced at the back of the oral cavity. Often, this is just a result of the beliefs of Anglophones or of non-speakers of those languages. Some languages which have fallen under the popular meaning of "guttural", as opposed to the technical meaning, are German, Ubykh, and Arabic.
French, Arabic, Welsh, Armenian, Hebrew, Scots, and also partly German, Dutch, Afrikaans, Portuguese, Somali, Yiddish all contain sounds that come from the back of the throat as well as some Northern English dialects. Sometimes whether a language is considered guttural or not could depend on differences within regions and countries. In French, the only truly guttural sound is a uvular trill; Arabic and Hebrew both contain rather more gutturals, including velar, uvular and pharyngeal fricatives.