How to remove folders with a certain name

If the target directory is empty, use find, filter with only directories, filter by name, execute rmdir:

find . -type d -name a -exec rmdir {} \;

If you want to recursively delete its contents, replace -exec rmdir {} \; with -delete or -prune -exec rm -rf {} \;. Other answers include details about these versions, credit them too.

Use find for name "a" and execute rm to remove those named according to your wishes, as follows:

find . -name a -exec rm -rf {} \;

Test it first using ls to list:

find . -name a -exec ls {} \;

To ensure this only removes directories and not plain files, use the "-type d" arg (as suggested in the comments):

find . -name a -type d -exec rm -rf {} \;

The "{}" is a substitution for each file "a" found - the exec command is executed against each by substitution.

This also works - it will remove all the folders called "a" and their contents:

rm -rf `find . -type d -name a`