R-dropping

R-dropping also called non-rhoticity or post vocalic r-lessness happens to English-speakers when the "r" sound is not pronounced after a vowel. Then the words "car" and "card"  both sound like "cah" and "cahd". That happens for some speakers in the United States, especially in the Boston and New York City areas and pockets of the American South especially the Greater New Orleans area, Virginia's Tidewater region and among speakers of ethnolects such as African American Vernacular English and Cajun English. British and Northeastern r-droppers usually pronounce "r" when the next word begins with a vowel as in "car is". For them, "spa" and "spar" sound alike, Southern, African American and South African r-droppers don't add additional Rs.

The pronunciation of the R is kept in the Scottish, Irish and most of the U.S. and Canada versions of spoken English. The R-droppers include most of England, Wales, Australia, New Zealand and South Africa.

Non-rhoticity can result in vowel mergers such as 

bud-bird merger - common in completely non-rhotic accents where the 'nurse' vowel merges with the short 'u'

comma-commer merger the r pronounced like a schwa at the end of a word.

dough-door merger - found mostly in r-dropping varieties of Southern American English and a few ethnolects such as Cajun English & AAVE. Results in the "or" dipthong being pronounced like the "oh" dipthong (examples include "floor" pronounced like "flow", " more" as "moe", " court" as "coat" etc)

The issue of how the R is used is called rhoticity by linguists.

 Other websites 
wiktionary for rhotic
wiktionary for non-rhotic

Category:Pronunciation