Ain't is an informal way to say: am not/ isn't/ aren't; hasn't/ haven't.
"Ain't has a long history of controversy. It first appeared in 1778, evolving from an earlier an't, which arose almost a century earlier as a contraction of are not and am not. In fact, ain't arose at the tail end of an era that saw the introduction of a number of our most common contractions, including don't and won't. But while don't and won't eventually became accepted at all levels of speech and writing, ain't was to receive a barrage of criticism in the 19th century for having no set sequence of words from which it can be contracted and for being a "vulgarism," that is, a term used by the lower classes, although an't had been originally used by the upper classes as well. At the same time ain't's uses were multiplying to include has not, have not, and is not, by influence of forms like ha'n't and i'n't. It may be that these extended uses helped fuel the negative reaction. Whatever the case, criticism of ain't by usage commentators and teachers has not subsided, and the use of ain't is often regarded as a sign of ignorance. · But despite all the attempts to ban it, ain't continues to enjoy extensive use in speech. Even educated and upper-class speakers see no substitute in folksy expressions such as Say it ain't so and You ain't seen nothin' yet. · The stigmatization of ain't leaves us with no happy alternative for use in first-person questions. The widely used aren't I?, though irregular, was found acceptable for use in speech by a majority of the Usage Panel as long ago as 1964, but in writing there is no acceptable substitute for the stilted am I not?" - http://www.thefreedictionary.com/ain't