[R] gc(), memory.size()

[r user]

Can someone please explain for me what the vcells and
ncells “used” column means when I run gc()?
> gc()
           used  (Mb) gc trigger  (Mb)  max used  
(Mb)
Ncells   882296  23.6   13812157 368.9  19400892 
518.1
Vcells 14811586 113.1  114763459 875.6 317464335
2422.1
>

(I read the help file , but still do not fully
understand?)
Also, how do I determine the total memory being
used?Do I simply run memory.size()?
Finally, when I run memory. size(max=T), I get a
negative value.  What does this mean