title: What are the double colons (::) in R?
body: <p>I am following a tutorial in Rbloggers and found the use of double colons, I looked online, but I couldn't find an explanation for their use. 
  Here is an example of their use.</p>
  
  <pre><code>df &lt;- dplyr::data_frame(
    year = c(2015, NA, NA, NA), 
    trt = c("A", NA, "B", NA)
  )
  </code></pre>
  
  <p>I understand it creates a data frame but I don't understand their purpose. </p>