Starting with Version 3.0, the Twenty Ten theme became the default (and fallback) theme. As described in 2010: A Theme Odyssey, the Twenty Ten theme serves as a good example theme that includes new theme-based features, and looks nice on a public site. Twenty Ten is a community-developed theme.
Up to Version 2.9.2, the default theme was the Kubrick theme and was housed in the wp-content/themes/defaultfolder. The Twenty Ten theme is housed in the wp-content/themes/twentyten folder and was the only theme in the WordPress distribution.