In everyday informal speech, most Americans don't pronounce the full "-ing". They pronounce "going" as "GO-in" rather than "GO-ing". It's just much easier to say "I'm goin' to the store." or "I'm thinkin' I'm gonna go to bed." We pronounce the "-ing" if we're speaking slowly, emphasizing the verb, or talking to someone with authority (a teacher, a boss, etc.) However, you should always write the "-ing". People who write " goin' " instead of "going" are either trying to transcribe the dialect, or seem "cool", like Michael mentioned. Other non-American English speakers do this too, particularly the Irish. Anyway, just pronounce the way that is easier for you. It's perfectly fine to use either "ing" or "in". No one will notice. However, always write it as "ing".

its slang. by not adding the 'g' its making it hip and cool, but its the same as with the full "ing"

Dialect. What it means is obvious.

there's no differences between goin' and going! Most people who pronounce them like goin' are from the south, Southern American English to be specific. Also, this kind of pronunciation is quite prevailing in African American Vernacular English! Hope this help ^^

Meaning the same thing as the last "g" is added. To make yourself sound "in". Also to make yourself sound stupid and pretentious if you precisely pronounce it as "goi-N", unless you are trying to be funny. Eclectically and neutrally, some Americans prefer a pronunciation between "going" and "goin", not belonging to either one, strictly. To make themselves sound kinda cowboy, lazy, Americanistic, uninterested, indifferent, vital, young, in, etc.