Unix or Linux Commands

Command Command Command Command
bash gcc man ps
cat ls

For descriptions of commands RTFM, which means do 

man command name
info command name

as appropriate. See also the the XKCD cartoon of this title.

To find the man page for a command you don't know the name of do 

man -k

as doing 

man man

would have told you.

R Functions

Function Function
lapply sapply

To RTFM for R itself do (of course, since R is a unix command) 

man R

for documentation of R functions, classes, and datasets, do (inside R) 

help(object name)
where object name is the name of the object. This works even for stuff that is not valid R objects if quoted, for example, 
help("[")
help("+")
help("for")

(help(for) wouldn't work because it doesn't parse, because for is an R keywork not a valid R object name).

The R analog of man -k is 

help.search(object name)


?foo
??foo
are abbreviations for 
help(foo)
help.search(foo)

respectively.