Httpd stands for Hyper Text Transfer Protocol Daemon (i.e. web server).

The implied meaning can be:

• The Apache HTTP Server.
• The Lighttpd HTTP server.
• The Nginx HTTP and reverse proxy server
• The NCSA HTTPd HTTP server.
• The CERN HTTPd HTTP server.
• The Null httpd HTTP server.
• The Thttpd HTTP server.
• The TUX web server aka kHTTPd
• The Canopy HTTPd HTTP server.

You might also want to check a Comparison of web servers