Builtins and Closures in R

I don't know , if this is the correct place to ask subjective questions and I am fairly new to R, but i am really confused at this time.I was going through R-Language reference and found two objects by running typeof(is.na) and typeof(mean) which returned "builtin" and "closure" respectively on R prompt. I don't know what does it mean, I went to this site after searching, http://www.r-bloggers.com/closures-in-r-a-useful-abstraction/ , but unable to understand , Can anyone help me in understanding "closure" and "builtin" in some layman terms?