httpd is the name of the web server executable on *nix systems. (When compiled for Windows, the executable is typically named Apache.exe or Apache2.exe.) By extension, the name refers to the entire suite of supporting software. The name stems from the first widely-used web server, httpd from the National Center for Supercomputing Applications (NCSA). This original httpd was developed with tax money in the public domain, and formed the original basis for Apache. The Apache project was originally a patched NCSA httpd, "a patchy" web server.
See also: Apache