The alveolar trill is a type of consonantal sound, used in some spoken languages. The symbol in the International Phonetic Alphabet that represents dental, alveolar, and postalveolar trills is ⟨r⟩, and the equivalent X-SAMPA symbol is r. It is commonly called the rolled R, rolling R, or trilled R. Quite often, ⟨r⟩ is used in phonemic transcriptions (especially those found in dictionaries) of languages like English and German that have rhotic consonants that are not an alveolar trill. This is partly due to ease of typesetting and partly because ⟨r⟩ is the letter used in the orthographies of these languages.
In the majority of Indo-European languages, this sound is at least occasionally allophonic with an alveolar tap, particularly in unstressed positions. Exceptions to this include Albanian, and a number of Armenian and Portuguese dialects, which treat them as completely separate phonemes.
Read more about Alveolar Trill: Features, Occurrence, Voiceless Alveolar Trill, Raised Alveolar Non-sonorant Trill