Old Galactic Standard started as a slang or jargon of traders and spacers, based on a mix of early Human languages, Durese, and Bothese. Because of its popularity with members of those professions its popularity and acceptance spread. Old Galactic Standard was later further influenced by Dromnyr, resulting ultimately in the standard Basic.
Under the Galactic Empire, a version of the language was known as Imperial Basic. The language was adopted by the Rebel Alliance, and as such was spoken in the Alliance of Free Planets, the New Republic, the Imperial Remnant, the Galactic Alliance, and the Fel, Sith and True Empires.
Most sentient species that made galactic contact could and did speak Basic in addition to whatever native or regional language they might have used on a daily basis. Some beings normally did not possess the organs necessary to speak Basic, including Polis Massans, Ssi-Ruuk, P'w'ecks, Wookiees, and a number of insectoid races. Most Gamorreans could not speak Basic, though there were exceptions (such as Governor Kugg.) Whether or not a being could speak Basic, it was usually in their best interest to comprehend it, and most did.
The Towanis taught the Ewoks a mysterious and undocumented language, which didn't help them understand Leia Organa, and later, Han Solo and Luke Skywalker, who spoke Basic. The relation of that language to Basic is not known.
High Galactic was a dialect (perhaps a vocabulary of alternative words) intended for displaying reverence and honor to religious figures.
Basic used the Aurebesh script.
Various cultures created different accents around the galaxy, each with their own unique set of pronunciations.
Of interesting note, the Wookiee Ralrracheen spoke with a speech impediment. Due to the differences in the vocal abilities of Ralrra, his dialect was perfectly understandable by non Wookiees, and it was further suggested that some Wookiees with this condition could speak Basic, at the loss of all forms of Shyriiwook.
Basic-speaking Rodians tended to suffer from speech impediments as well, most notably a lisp. Avaro Sookcool and Teeloo were two such Rodians; the syllables R, L, and S proved to be the most problematic to vocalize.
In both the films and books, Basic is, with the exception of some—still, English-based—words and phrases devised by authors, the same as English. This could mean two things: Either Galactic Basic actually is the same as the English language used today on Earth, or it is translated as English in a manner similar to Westron in J. R. R. Tolkien's Lord of the Rings. Languages of the Star Wars universe like Ewokese in the animated series and the language spoken by the Towanis in Caravan of Courage: An Ewok Adventure and Ewoks: The Battle for Endor have been translated to English in this manner.
In translations to non-English languages, Basic is assumed to be English but translated to the other language.
In E.T.: The Book of the Green Planet, a character mentions that she needs help with her "basic". However, in the context of the book, "basic" is most likely a reference to the computer programming language known by that name.
While the letters are different in Basic than English, there are multiple examples where latin alphabet letters are mentioned, such as Obi-Wan-Kenobi's quote "Do you have a plan B?", Misson Vao's habitual reference to Zaalbar as "Big Z", and most droid designations. Greek letters are used at times, examples including Delta Squad.
Dialects in English correspond to different in-universe accents in the movies. Ewan McGregor's and Ian McDiarmid's faint upper-class Scottish accents are, for example, Coruscanti accents in the movies. As a general rule, Imperial characters have English accents while Rebels usually have Californian accents. However, it would seem that this is perhaps a representation of social class, as stormtroopers and other low-ranking Imperials are heard to speak with American accents, and some Rebels in English accents (Mon Mothma, for instance). Other accents are also heard, such as:
Star Wars authors have coined original English-like words derived from real English elements but which don't exist in the English language; they seem useful to a space-driven civilization of the Star Wars characters, but totally useless for our daily language. Such words are 'speciesism' (equivalent to racism, but against sentient beings other than the speakers), 'gentlebeing' (equivalent to gentleman, but applicable to all species, including non-humanoid ones), 'offworlder', 'bird' (slang for spaceship, cf. how US Navy slang calls 'torpedo' as 'fish'), 'hyperspace', 'homeworld', 'spacer', 'sentientologist', 'youngling' (young + suffix -ling meaning 'person') and other technological terms like blaster, midi-chlorian, droid (from android), durasteel, astrogation etc.
However, Basic vocabulary includes some totally original, of unexplainable etymology, words also unrelated to English, like Moff, Jedi, Padawan, Darth etc. or even compounds like Twi'lek ("twin lekku"). It is possible that such words are derived from other languages.