What is Turtle in the World of Semantic Web?

Turtle is a type of notation and a W3C Recommendation since February 2014.  

Its full name is Terse RDF Triple Language. 

It is one of a family of ways to write RDF, also known as RDF serialization formats. Turtle's plus point is that it is user-friendly (and less verbose, than say, XML).

Turtle's official documentation states that it:

 "allows an RDF graph to be completely written in a compact and natural text form, with abbreviations for common usage patterns and datatypes. Turtle provides levels of compatibility with the N-Triples [N-TRIPLES] format as well as the triple pattern syntax of the SPARQL W3C Recommendation".