merge(x,y,by=intersect(names(x),names(y)),
by.x=by, by.y=by, all=FALSE, all.x=all, all.y=all,
sort=TRUE, suffixes=c(".x",",y"),
incomparables=NULL...)

Explanations about function merge():
x.y : the two data frames used to be combined;
by,by.x, by.y:  to designate on which cols the merge is based; the default value is the cols share the same names;
all, all.x, all.y : appoint whether all the rows of x and y should be in outpu;
sort :  whether to sort the cols designated;
suffixes: designate the suffixes of the same-name cols other than by;
incomparables: designate which elements in "by" are not included in the merge;