RDF is a general purpose language used for representing info on the web. It primarily is used for inputting metadata. A definition for metadata can easily be found but rather than a definition an example is easier to understand. Suppose you find an image of the Sistine Chapel online. The data is what it is, the Sistine chapel represented digitally. The metadata is all the information about the image such as: who was the creator of this image, what type of work was it, what medium was it done in, where is the repository for this image, what format is the image and so on. That information is metadata.

RDF is the language used to input such information regarding things of the web, such as webpages, and web resources. In RDF, information is simply a collection of statements, each with a subject, verb and object. In Notation 3, of which Turtle is somewhat a derivative, one can write such a triple just as is with a period. For instance,

<#pat> <#knows> <#jo> . 

Turtle which stands for Terse RDF Triple Language is a solution proposed by the W3C team for XML/RDF code which is quite verbose and a pain to write manually.