In phonetics, retroflection refers to the curling back of the tip of the tongue to produce sounds. Retroflection is very common in Indian languages, most notably Hindi, and is often heard in stereotypes of these languages.
Retroflection can occur on any sound between the dental/alveolar point of articulation and the hard palate. This roughly means the sounds /t/,/d/,/s/,/ts/,/dz/,/š/,/ž/,/tj/,/dj/,/š/,/ž/,/n/,/l/,/r/ and many, many others (this area of articulation is the most common among the world's languages), including the clicks that are so prevalent in Zulu and other South African languages, and the glottalized versions of these sounds that are common in Maya languages.Also, vowels can be retroflexed.
In English, the sound 'r' is often retroflexed in many dialects, especially American dialects. It should be noted that this sound, which is almost technically a vowel, is one of the most difficult sounds for non-English speakers to master, and is different form the 'r's of other European languages.
Retroflection causes another process, called rhotacization, or r-coloring to occur, which gives all surrounding sounds a distinct 'r-ishness.' In many dialects of American English, this happens on the consonants 't' and 'd' before vowels, causing them to sound a bit like our 'ch'(/š/) and 'j'(/dž/), with a bit of r-ish sound.
In IPA, retroflection is represented by drawing a small hook on the bottom or botton right side of the symbol, with the opening of the hook facing right.