Westron, also known as the Common Speech, is the closest thing to the universal language of Middle-earth; at least during the War of the Ring. Westron is an English word, derived from the word West, not a word from the language itself.
The Westron speech was derived from the Adûnaic tongue of Númenor, and originated as a Creole language on the western coastlands of the continent of Middle-earth, when the Númenorians established trade outposts and forts there. From there, it spread east, with the notable exception of Mordor.
Westron was a translation of the original name Adûni, and "Common Speech" translates the Westron term Sôval Phârë, of identical meaning. In Sindarin the language was called Annúnaid (Westron), or Falathren (Shore-language).
In The Hobbit and The Lord of the Rings, Westron was presented as having been completely translated by English. This had certain important implications: first of all, proper names with derivations somewhat evident to speakers of Westron had been translated, to preserve the effect. Thus, names like Baggins, Bagshot Row, Peregrin, Rivendell etc., are presented as not the actual names. (For example, Meriadoc Brandybuck's actual name is supposed to have been Kalimac Brandagamba, short Kali (meaning jolly, merry). 'Meriadoc', short 'Merry', is designed to maintain the reference to merriness contained in the original name. Likewise Peregrin Took's actual name was Razanur Tûc, short Razar (name of a small apple). 'Peregrin', short 'Pippin' contained both the actual meaning of the full name (traveller, stranger) and the reference to an apple). Sam Gamgee was actually named Ban Galpsi, short for Banazir Galbasi. The ending of the 'true' Hobbit name Bilbo was also changed: in Westron it was Bilba, but Tolkien changed this to -o because -a is usually a female ending in English.
Placenames and other features were also presented as having been translated from an original form: Rivendell (Sindarin Imladris, "cloven valley") was actually called Karningul, and Bag End was actually called Labin-nec, after Labingi, the real form of Baggins. In some cases the explanations became quite involved, such as the river Brandywine (Sindarin Baranduin, "golden-brown river") was actually called Branda-nîn, a punning Westron name meaning "border-water", which was later punned again as Bralda-hîm meaning "heady ale".
The translation went one step further by also changing all languages akin to Westron. Rohirric, the language of the Rohirrim was translated by Anglo-Saxon, as Rohirric is an archaic relative of Westron (since the Edain from whose speech Westron is derived were related to the ancestors of the Rohirrim) much as Anglo-Saxon is an archaic relative of English. Similarly, the tongue of Dale, from which came the names of the Dwarves of Durin's house, was translated by Old Norse, a language related to Anglo-Saxon and modern English as Dalish was related to Rohirric and Westron.
This utter translation of Westron by English was taken so far that some sources that should give actual Westron have been turned to English too. For instance, in Moria, an illustration of the runic text on Balin's gravestone is given. The text is said to mean "Balin Son of Fundin, Lord of Moria" in both Khuzdûl and Westron... but while the first part of the inscription seems to really be a bit of Khuzdûl (Moria is rendered as "Khazad-dûm"), the second part is actually plain English, just written in certar.
Outside the context of the story, it is clear that most of the "original" forms in Westron or other languages were devised by Tolkien long after the English "translations" were chosen. Several of the Westron forms given above were not published in Tolkien's lifetime. Tolkien never worked out Westron to the same extent as Quenya and Sindarin or even Adûnaic.