Depending on your disto there is probably an init script either in /etc/rc, as cdlvj states, or in /etc/init.d. On a Redhat/Fedora system you can control the initscripts with "/sbin/service httpd start" or "/sbin/service httpd restart", etc.
These scripts usually call apachectl, but I point out the init scripts because sometimes they have additional configuration that could get overlooked. For example, "apachectl restart" is the equivalent of "apachectl stop ; apachectl start", but our webserver requires "apachectl startssl". "service httpd restart" takes that into account and does The Right Thing(tm).