Is there any difference between apache2 and httpd?

httpd is the same as apache2. It depends on the OS you use. For example in RHEL 6.2 it is called httpd and in Ubuntu it is called apache2.

TL;DR

They are the same application - just that some Linux distributions refer to it differently within package managers and config files. RedHat-based distros (CentOS, Fedora) refer to it as httpd while Debian-based distros (Ubuntu) refer to it as apache. Gentoo, strangely enough, mostly refers to it as apache - but config files have httpd in the naming conventions.

Background

The application's development is managed by the Apache Foundation. Its name is actually Apache HTTP Server. It is often also called Apache httpd (http daemon) by the Apache Foundation. Colloquially, Unix admins have historically called it Apache and, even with the RedHat-based distributions, everyone already knows what you're referring to when you simply say Apache.

The directives used in apache2 and httpd config files are slightly different, as well as the directory structure. It's advised to install apache2 on Debian-based distros (Ubuntu) and httpd on RedHat-based distros (RedHat, CentOS, Fedora).

However, if you install httpd and its dependencies on Ubuntu, it works as well.