The logic of
plyr is very similar to
apply(). In fact, the title of the package is Tools for splitting, applying, and combining data.
Its main functions can be easily understood since they are all named for its input and output objects, based upon the following references:
d: This is for a data frame
a: This is for an array
l: This is for a list
m: This is for a data frame or an array (column-wise, only as input)
_: This is for a function's guess (only for outputs)
So, for instance,
laply receives a list and returns an array,
ddply receives a data frame and returns a data frame, and so on.
Although the package has a wide variety of functions available, all the ones that have a data frame as input are the most important ones (also, ...