Using xargs to redirect to command line arguments

To delete all .bak files in a directory, do
rm *.bak or ls *.bak | xargs rm
Essentially, rm is executed once with the whole of the piped input as argument.

To move the .bak files to another place, do
mv *.bak diffdir or ls *.bak | xargs -I var1 mv var1 diffdir.
Here, mv is executed separately for each file listed by ls. var1 is a placeholder. It is replaced each time by a file, and mv is executed.

To rename the .bak files to .bakup files, do
ls *.bak | xargs -0 -I {} mv {} {}up.
{} is another way of specifying a placeholder. -0 takes care of white space anomalies in the piped input.