If no name is specified on the command line, then du reports the usage at the current directory. The default behavior of du is as follows. The disk usage is displayed at each directory encountered. The displayed usage includes the accumulated contents of that directory. Command line switches are available to modify nearly every aspect of this default behavior.
Disk usage is reported as a number of blocks.
The size of these blocks is site-specific, but usually is either 512 bytes or 1024 bytes. Use the -h option to see how your version of du is configured. There are a number of options that will select a different block size for reporting disk usage. These are not the same thing, and, unfortunately, on most modern systems the sizes of the two are different. Once upon a time, a long time ago, the Unix filesystem storage block size was 512 bytes, and du reported usage in 512-byte blocks.