[R] builtin vs. closure

[Edna Bell]

Dear R Gurus:

I'm working slowly through "R Programming for Bioinformatics", which
is really interesting!

Anyway, my question now is:  what determines if a function is a
builtin vs. a closure, please?

For instance:
> typeof(sqrt)
[1] "builtin"
> typeof(mean)
[1] "closure"
>

Thanks,
Edna Bell