Braces in general are great for all sorts of tasks. The general principle is that it duplicates arguments, with changes. Thus,

  foo/bar/{a,b,c}

expands out to

  foo/bar/a foo/bar/b foo/bar/c

and you can also use {1..9} to expand to all the numbers between 1 and 9 (inclusive).

Here are some ways I commonly use it:

- Create lots of subdirectories at once:

  mkdir -p test/{a,b,c}/{1..4}

- Rename/copy files (as in parent comment)

- Removing lots of randomly named files, with common elements:

  rm {a,d,f,gr}*

(this will remove all files starting with those prefixes...note that it will not delete other files starting with 'g', only those with 'gr')