The du (or 'disk usage') command is a Linux command that can print a list of the files within a directory including their sizes and even summarize this information. It is useful if you want to see how large a group of files is and provides more information about directories than the ls command does.
Using du within a directory will show you the size (in bytes) of all files and directories under that directory, including the size of the current directory. To make du produce more readable results just use the -h flag to make the file sizes into a human readable format. You can also use the -c flag to produce a grand total of all of the sizes found, and the -s flag to display only a total. The -a flag can be used to display all files as well as directories, leave this out to just display directories.