du - Summarize Disk Usage

du will list the number of 1K blocks used in the current directory and each subdirectory under it.
du in the root directory will do this for all subdirectories - use with the more or less pipelines

Options include:
-a - Show counts for all files, not just directories
-b - Show sizes in bytes
-c - Produce a grand total
-h - "Human Readable" format - shows sizes as 1K, 23M, 4G
-x - Skip directories on different file systems (ex excludes CD-ROM)

Note that du of the root directory will also list the other mounted disk drives including floppies and CD-ROM's) unless you use du -x (one file system)

du -h *.cpp - will list all files ending with .cpp in the current directory.

see df to summarize disk free space