Sorich87 is right. The WP Codex describes a contributor as:
Somebody who can write and manage their posts but not publish them. (See Roles and Capabilities). If you want people to automatically have this capability when they register, you can set the default role to Contributor in the settings panel.
If you need additional permissions not handled by the built in WordPress permissions — say, for example, you want a user that can edit other people’s posts as well, but once they’re edited they go back to being drafts, or whatever — then there are several good Role Manager plugins. For more information see the documentation on Roles and Capabilities Plugin.