An IDE is an "Integrated Development Environment". An IDE application contains most, if not all, of the tools that developers need all within a single application. If you have only developed with a text editor in the past, you will find that Eclipse contains much more functionality, including editing, code validation, file and project management, and debugging. The plugin architecture means you can extend the functionality for your own needs.
Aptana Studio provides robust support for HTML, CSS, and JavaScript through added web development tools, such as code assist, popular web application development language support, database tools and FTP/SFTP.
Aptana Studio and its related plugins such as RadRails, PHP and Pydev are available as standalone applications or as plugins to Eclipse. To learn more about how plug-ins work with Eclipse and Eclipse-based development environments, see About plug-ins.
Both Eclipse and Aptana are free and open source.