An addendum to my earlier post:
The command chmod 755 * will change the permissions of all files and directories in the current directory to rwx-r-x-r-x. However, you may have had something else in mind, such as changing all files and directories in an entire tree to a specified permission. You'll need a command such as the following to do this:
find . -exec chmod 755 {} \;
which will change all files and directories in and below the current directory to permission rwx-r-x-r-x. You can restrict the changes to directories by adding the -type parameter to the find command:
find . -type d -exec chmod 755 {} \;
You can use:
find . -type f -exec chmod 755 {} \;
to apply the changes only to files. Note that you must own the file or directory or be the superuser in order to change its mode.
Hope this helps.