Why do many processes use the letter "d" at the end of the process name?
Since OS X is in part based on FreeBSD here is relevant documentation from the FreeBSD Handbook's 3.8. Processes and Daemons...


  "There is a convention to name programs that normally run as daemons
  with a trailing “d”. For example, BIND is the Berkeley Internet Name
  Domain, but the actual program that executes is named. The Apache web
  server program is httpd and the line printer spooling daemon is lpd.
  This is only a naming convention. For example, the main mail daemon
  for the Sendmail application is sendmail, and not maild."


Take note of the last two sentences from what I quoted, the trailing "d" is only a convention but as you can see with the example sendmail is the mail daemon and doesn't have the trailing "d".  There are also other daemons in Unix/Unix like OSes that do not use the trailing "d" for daemons however as a convention it is a good practice but not an absolute.