Like is one of the words in the English language that can introduce a simile (a stylistic device comparing two dissimilar ideas) as in, "He plays like Okocha". It can also be used in non-simile comparisons such as, "She has a dog like ours".
Many people became aware of the two options in 1954, when a famous ad campaign for Winston cigarettes introduced the slogan "Winston tastes good — like a cigarette should." The slogan was criticized for its usage by prescriptivists, the "as" construction being considered more proper. Winston countered with another ad, featuring a woman with greying hair in a bun who insists that ought to be "Winston tastes good as a cigarette should" and is shouted down by happy cigarette smokers asking "What do you want — good grammar or good taste?"
The appropriateness of its usage as a conjunction is still disputed, however. In some circles, it is considered a faux pas to use like instead of as or as if, whereas in other circles as sounds stilted.
As a noun
Like can be used as a noun meaning "preference" or "kind". Examples:
She had many likes and dislikes.
We'll never see the like again.
As a verb
As a verb, like generally refers to a fondness for something or someone. Example:
I like riding my bicycle.
Like can be used to express a feeling of attraction between two people, weaker than love and distinct from it in important ways. It does not necessarily imply a romantic attraction. Example:
Marc likes Denise.
Torbjørn likes Kristian.
As a colloquial adverb
In some regional dialects of English, like may be used as an adverbialcolloquialism in the construction be + like + toinfinitive, meaning "be likely to, be ready to, be on the verge of."
He was like to go back next time.
He was like to go mad.
As the following attest, this construction has a long history in the English language.
But Clarence had slumped to his knees before I had half-finished, and he was like to go out of his mind with fright. (Mark Twain, 1889, Connecticut Yankee in King Arthur's Court)
He saw he was like to leave such an heir. (Cotton Mather, 1853, Magnalia Christi Americana)
He was like to lose his life in the one [battle] and his liberty in the other [capture], but there was none of his money at stake in either. (Charles MacFarlane and Thomas Napier Thomson, 1792, Comprehensive History of England)
He was in some fear that if he could not bring about the King's desires, he was like to lose his favor. (Gilbert Burnet, 1679, History of the Reformation of the Church of England)
As a colloquial quotative
Like is sometimes used colloquially as a quotative to introduce a quotation or impersonation. This is also known as "quotation through simile". The word is often used to express that what follows is not an exact quotation but instead gives a general feel for what was said. In this usage, like functions in conjunction with a verb, generally be (but also say, think, etc.), as in the following examples:
He was like, "I'll be there in five minutes."
She was like, "You need to leave the room right now!"
Like can also be used to paraphrase an implicitly unspoken idea or sentiment:
They told me all sorts of terrible things, and I'm like "Forget it then."
It is also sometimes used to introduce non-verbal mimetic performances, e.g., facial expressions, hand gestures, body movement, as well as sounds and noises:
I was like [speaker rolls eyes].
The car was like, "vroom!"
Linguist Patricia Cukor-Avila jokingly states that "eventually all the people who hate this kind of thing are going to be dead, and the ones who use it are going to be in control."
As a discourse particle, filler, hedge, or speech disfluency
History of this usage
The word like has developed several non-traditional uses in informal speech. Especially since the late 20th century onward, it has appeared, in addition to its traditional uses, as a colloquialism across all dialects of spoken English, serving as a discourse particle, filler, hedge, speech disfluency, or other metalinguistic unit. Although these particular colloquial uses of like appear to have become widespread rather recently, its use as a filler is a fairly old regional practice in Welsh English and, in Scotland, it was used similarly at least as early as the 19th century. It may also be used in a systemic format to allow individuals to introduce what they say, how they say and think.
Despite such prevalence in modern-day spoken English, these colloquial usages of like rarely appear in writing (unless the writer is deliberately trying to replicate colloquial dialogue) and they have long been stigmatized in formal speech or in high cultural or high social settings. Furthermore, this use of like seems to appear most commonly, in particular, among natively English-speaking children and adolescents, while less so, or not at all, among middle-aged or elderly adults. One suggested explanation for this phenomenon is the argument that younger English speakers are still developing their linguistic competence, and, metalinguistically wishing to express ideas without sounding too confident, certain, or assertive, use like to fulfill this purpose.
In pop culture, such colloquial applications of like (especially in verbal excess) are commonly and often comedically associated with Valley girls, as made famous through the song "Valley Girl" by Frank Zappa, released in 1982, and the film of the same name, released in the following year. The stereotyped "valley girl" language is an exaggeration of the variants of California English spoken by younger generations.
This non-traditional usage of the word has been around at least since the 1950s, introduced through beat and jazz culture. The beatnik character Maynard G. Krebs (Bob Denver) in the popular Dobie Gillis TV series of 1959-1963 brought the expression to prominence; this was reinforced in later decades by the character of Shaggy on Scooby-Doo (who was based on Krebs).
Very early use of this locution can be seen in a New Yorker cartoon of 15 September 1928, in which two young ladies are discussing a man's workplace: "What's he got - an awfice?" "No, he's got like a loft."
It is also used in the 1962 novel A Clockwork Orange by the narrator as part of his teenage slang and in the Top Cat cartoon series from 1961 to 1962 by the jazz beatnik type characters.
'What'll like be your business, mannie?'
'What's like wrong with him?' said she at last.
Like can be used as hedge to indicate that the following phrase will be an approximation or exaggeration, or that the following words may not be quite right, but are close enough. It may indicate that the phrase in which it appears is to be taken metaphorically or as a hyperbole. This use of like is sometimes regarded as adverbial, as like is often synonymous here with adverbial phrases of approximation, such as "almost" or "more or less."
I have, like, no money.
The restaurant is only, like, five miles from here.
I, like, died!
They, like, hate you!
In the UK Reality Television Series Love Island the word 'Like' has been used an average of 300 times per episode, much to the annoyance of viewers. 
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^Quoted from: Daniel P. Cullen, "I'm Learning as I Go, and I Don't Like That": Urban Community College Students' College Literacy, ProQuest 2008, p. 210.