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What is H and explain in detail?
H, or h, is the eighth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is aitch (pronounced , plural aitches), or regionally haitch .

History
The original Semitic letter Heth most likely represented the voiceless pharyngeal fricative (ħ). The form of the letter probably stood for a fence or posts.
The Greek Eta Η in archaic Greek alphabets, before coming  to represent a long vowel, /ɛː/, still represented a similar sound, the voiceless glottal fricative /h/. In this context, the letter eta is also known as Heta to underline this fact. Thus, in the Old Italic alphabets, the letter Heta of the Euboean alphabet was adopted with its original sound value /h/.
While Etruscan and Latin had /h/ as a phoneme, almost all Romance languages lost the sound—Romanian later re-borrowed the /h/ phoneme from its neighbouring Slavic languages, and Spanish developed a secondary /h/ from /f/, before losing it again; various Spanish dialects have developed [h] as an allophone of /s/ or /x/ in most Spanish-speaking countries, and various dialects of Portuguese use it as an allophone of /ʀ/. H is also used in many spelling systems in digraphs and trigraphs, such as ch, which represents /tʃ/ in Spanish, Galician, and Old Portuguese; /ʃ/ in French and modern Portuguese; /k/ in Italian and French.

Name in English
For most English speakers, the name for the letter is pronounced as  and spelled "aitch" or occasionally "eitch". The pronunciation  and the associated spelling "haitch" is often considered to be h-adding and is considered non-standard in England. It is, however, a feature of Hiberno-English, and occurs sporadically in various other dialects.
The perceived name of the letter affects the choice of indefinite article before initialisms beginning with H: for example "an H-bomb" or "a H-bomb". The pronunciation /heɪtʃ/ may be a hypercorrection formed by analogy with the names of the other letters of the alphabet, most of which include the sound they represent.The haitch pronunciation of h has spread in England, being used by approximately 24% of English people born since 1982, and polls continue to show this pronunciation becoming more common among younger native speakers. Despite this increasing number, the pronunciation without the /h/ sound is still considered to be standard in England, although the pronunciation with /h/ is also attested as a legitimate variant. In Northern Ireland, the pronunciation of the letter has been used as a shibboleth, with Catholics typically pronouncing it with the /h/ and Protestants pronouncing the letter without it.Authorities disagree about the history of the letters name. The Oxford English Dictionary says the original name of the letter was [ˈaha] in Latin; this became [ˈaka] in Vulgar Latin, passed into English via Old French [atʃ], and by Middle English was pronounced [aːtʃ]. The American Heritage Dictionary of the English Language derives it from French hache from Latin haca or hic. Anatoly Liberman suggests a conflation of two obsolete orderings of the alphabet, one with H immediately followed by K and the other without any K: reciting the formers ..., H, K, L,... as [...(h)a ka el ...] when reinterpreted for the latter ..., H, L,... would imply a pronunciation [(h)a ka] for H.

Use in writing systems
English
In English, ⟨h⟩ occurs as a single-letter grapheme (being either silent or representing the voiceless glottal fricative () and in various digraphs, such as ⟨ch⟩ , , , or ), ⟨gh⟩ (silent, /ɡ/, /k/, /p/, or /f/), ⟨ph⟩ (/f/), ⟨rh⟩ (/r/), ⟨sh⟩ (), ⟨th⟩ ( or ), ⟨wh⟩ (/hw/). The letter is silent in a syllable rime, as in ah, ohm, dahlia, cheetah, pooh-poohed, as well as in certain other words (mostly of French origin) such as hour, honest, herb (in American but not British English) and vehicle (in certain varieties of English). Initial /h/ is often not pronounced in the weak form of some function words including had, has, have, he, her, him, his, and in some varieties of English (including most regional dialects of England and Wales) it is often omitted in all words (see ⟨h⟩-dropping). It was formerly common for an rather than a to be used as the indefinite article before a word beginning with /h/ in an unstressed syllable, as in "an historian", but use of a is now more usual (see English articles § Indefinite article). In English, The pronunciation of ⟨h⟩ as /h/ can be analyzed as a voiceless vowel. That is, when the phoneme /h/ precedes a vowel, /h/ may be realized as a voiceless version of the subsequent vowel. For example the word ⟨hit⟩, /hɪt/ is realized as [ɪ̥ɪt]. H is the eighth most frequently used letter in the English language (after S, N, I, O, A, T, and E), with a frequency of about 4.2% in words. When h is placed after certain other consonants, it modifies their pronunciation in various ways, e.g. for ch, gh, ph, sh, and th.

Other languages
In the German language, the name of the letter is pronounced /haː/. Following a vowel, it often silently indicates that the vowel is long: In the word erhöhen (heighten), the second ⟨h⟩ is mute for most speakers outside of Switzerland. In 1901, a spelling reform eliminated the silent ⟨h⟩ in nearly all instances of ⟨th⟩ in native German words such as thun (to do) or Thür (door). It has been left unchanged in words derived from Greek, such as Theater (theater) and Thron (throne), which continue to be spelled with ⟨th⟩ even after the last German spelling reform.
In Spanish and Portuguese, ⟨h⟩ ("hache" in Spanish, pronounced [atʃe], or agá in Portuguese, pronounced [aˈɣa] or [ɐˈɡa]) is a silent letter with no pronunciation, as in hijo [ˈixo] (son) and húngaro [ˈũɡaɾu] (Hungarian). The spelling reflects an earlier pronunciation of the sound /h/. In words where the ⟨h⟩ is derived from a Latin /f/, it is still sometimes pronounced with the value [h] in some regions of Andalusia, Extremadura, Canarias, Cantabria, and the Americas. Some words beginning with [je] or [we], such as hielo, ice and huevo, egg, were given an initial ⟨h⟩ to avoid confusion between their initial semivowels and the consonants ⟨j⟩ and ⟨v⟩. This is because ⟨j⟩ and ⟨v⟩ used to be considered variants of ⟨i⟩ and ⟨u⟩ respectively. ⟨h⟩ also appears in the digraph ⟨ch⟩, which represents /tʃ/ in Spanish and northern Portugal, and /ʃ/ in varieties that have merged both sounds (the latter originally represented by ⟨x⟩ instead), such as most of the Portuguese language and some Spanish dialects, prominently Chilean Spanish.
In French, the name of the letter is written as "ache" and pronounced /aʃ/. The French orthography classifies words that begin with this letter in two ways, one of which can affect the pronunciation, even though it is a silent letter either way. The H muet, or "mute" ⟨h⟩, is considered as though the letter were not there at all, so for example the singular definite article le or la, which is elided to l before a vowel, elides before an H muet followed by a vowel. For example, le + hébergement becomes lhébergement (the accommodation). The other kind of ⟨h⟩ is called h aspiré ("aspirated ⟨h⟩", though it is not normally aspirated phonetically), and does not allow elision or liaison. For example in le homard (the lobster) the article le remains unelided, and may be separated from the noun with a bit of a glottal stop. Most words that begin with an H muet come from Latin (honneur, homme) or from Greek through Latin (hécatombe), whereas most words beginning with an H aspiré come from Germanic (harpe, hareng) or non-Indo-European languages (harem, hamac, haricot); in some cases, an orthographic ⟨h⟩ was added to disambiguate the [v] and semivowel [ɥ] pronunciations before the introduction of the distinction between the letters ⟨v⟩ and ⟨u⟩: huit (from uit, ultimately from Latin octo), huître (from uistre, ultimately from Greek through Latin ostrea).
In Italian, ⟨h⟩ has no phonological value. Its most important uses are in the digraphs ch /k/ and gh /ɡ/, as well as to differentiate the spellings of certain short words that are homophones, for example some present tense forms of the verb avere (to have) (such as hanno, they have, vs. anno, year), and in short interjections (oh, ehi).
Some languages, including Czech, Slovak, Hungarian, Finnish, and Estonian use ⟨h⟩ as a breathy voiced glottal fricative [ɦ], often as an allophone of otherwise voiceless /h/ in a voiced environment.
In Hungarian, the letter has no fewer than five pronunciations, with three additional uses as a productive and non-productive element of digraphs