Here are some of the ways that words are gained, lost and changed in the English language.
Commonisation.
Commonisation occurs when something that was previosuly a name, loses its capital letter and becomes a common, household word. For example the word lamington, name of the chocolate covered pieces of sponge cake rolled in coconut, came from Baron Lamington, a previous Governor of Queensland. Jeans are also an example of commonisation, as they are named after the town of Genoa, where a heavy fabric closely resembling denim was made.
Acronyms and initialisms.
Acronyms and initialisms (previously known as abbreviations) are two closely linked but seperate concepts. The term acronym refers to words that are formed from the initials of other words, but are spoken as a word and not as a series of letters, examples of acronyms include LOL for 'laugh out loud' and DOS for 'disk operating system'. Initialisms are also formed from the initials of words, but rather than be spoken as a word, initialisms are spoken as a sequence of letters, like BRB for 'be right back' and BO for 'body odour'.
Blends.
Blends are words that are created from the joining of two different words, incorporating the meanings of both of them. 'Cracker' for 'credit card hacker' and 'foreploy' for 'any misunderstanding or outright lie that leads to sex' are great examples of this phenomenon.
Shortenings.
Shortenings are the shortened, or truncated versions of longer words, that come to replace the older, long versions entirely and become their own words. Sometimes the two words become entirely distanced from eachother and the connection is hard to spot, like 'stroppy' from 'obstreperous' and 'grotty' from 'grotesque'.
Borrowing.
The ways in which language gains new words not only draw on sources within the language, but also outside of it, in the form of 'borrowing'. English is one of the largest and most notorious borrowing languages with words from over 160 different languages, includign large amounts from the languages of our own indigenous australians. From various aboriginal languages we have gained words for many things, but primarily names for animals, plants and places. For example boomerang, jarrah, waratah, coolibah, galah, kookaburra, dingo, wallaby, wombat and kangaroo are all indiginous words.