In English, ll represents the same sound as single l: /l/. The doubling is used to indicate that the preceding vowel is (historically) short, or for etymological reasons, in latinisms (coming from a gemination).
Digraph considered since 1803 to be the fourteenth letter of the Spanish alphabet because of its representation of a palatal lateral articulation consonant phoneme. In great part of the Hispanic countries and regions it is pronounced as a "y", with central emission of air, and with the same variations of articulation. (definition by the Royal Academy of the Spanish Language)
However, nowadays most Spanish speakers pronounce ll the same as y (yeísmo). As a result, in most parts of Hispanic America as well as in many regions of Spain, Spanish speakers pronounce it /ʝ/ (voiced palatal fricative), while some other Hispanic Americans (especially Rioplatense speakers, and in Tabasco, Mexico) pronounce it /ʒ/ (voiced postalveolar fricative) or /ʃ/ (voiceless postalveolar fricative).
This digraph was considered a single letter in Spanish orthography, called elle. From 1803 it was collated after L as a separate entry, a practice now abandoned: in April 1994, a vote in the X Congress of the Association of Spanish Language Academies ruled the adoption of the standard Latin alphabet collation rules, so that for purposes of collation the digraph ll is now considered a sequence of two characters. The same is now true of the Spanish-language digraph ch. Hypercorrection leads some to wrongly capitalize it as a single letter (*"LLosa" instead of the official "Llosa"; "LLOSA" is the right form in full uppercase) as with the Dutch IJ. In handwriting, it is written as a ligature of two L's, with a distinct uppercase and lowercase form. An old ligature for Ll is known as the "broken L", which takes the form of a lowercase l with the top half shifted to the left, connected to the lower half with a thin horizontal stroke. This ligature is encoded in Unicode at U+A746 (uppercase) and U+A747 (lowercase) and displayed (by the browsers that allow it) Ꝇ and ꝇ respectively.
In official Galician spelling the ll combination stands for the phoneme /ʎ/ (palatal lateral approximant, a palatal counterpart of /l/). Galician language, as Portuguese language, writes grapheme <y> only for loanwords, and thus doesn't present a unified pronunciation (sometimes a consonant like ll or sometimes a vowel like i for words when y is intervocalic). Almost all Galician natives speak doesn't distinguish this consonants and speak with yeismo, being classified under this characteristic for both Galician and Spanish.
In Catalan, ll represents the phoneme /ʎ/. For example, as in llengua "language" or "tongue", enllaç "linkage", "connection" or coltell "knife". In order to not confuse ll /ʎ/ with a geminated l /ll/, the ligature ŀl is used with the second meaning. For example, exceŀlent is the Catalan word for "excellent", from Latin excellente. In Catalan, l·l must occupy two spaces, so the interpunct is placed in the narrow space between the two L: ĿL and ŀl. However, it is more common to write L·L and l·l, occupying three spaces; this practice is not correct although it is tolerated. L.L and l.l are incorrect and not accepted. See interpunct for more information.
While Philippine languages like Tagalog and Ilokano write ly or li in the spelling of Spanish loanwords, ll still survives in proper nouns. However, the pronunciation of ll is simply [lj] rather than [ʎ]. Hence the surnames Llamzon, Llamas, Padilla and Villanueva are respectively pronounced [ljɐmˈzon]/[ljɐmˈson], [ˈljɐmas], [pɐˈdɪːlja] and [ˌbɪːljanuˈwɛːba]/[ˌvɪːljanuˈwɛːva].
In Welsh, ll stands for a voiceless lateral fricative sound. The IPA signifies this sound as ɬ. This sound is very common in place names in Wales because it occurs in the word Llan, meaning "parish" or "church of Saint ...", for example, Llanelli, where the ll appears twice, or the invented place-name Llanfairpwllgwyngyllgogerychwyrndrobwyllllantysiliogogogoch, where the ll appears five times. These Welsh place names therefore very often bear simplified pronunciations in English (generally the ll sound being replaced by chl (the ch pronounced as in loch)).
In Welsh dictionaries, LL is treated as a separate letter from L (e.g. lwc sorts before llaw). This led to its ligature being included in the Latin Extended Additional Unicode block. The capital ligature appears similar to a joined "IL" and the minuscule ligature like "ll" joined across the top.