Appcelerator, Titanium and Aptana Studio are based on the Eclipse platform and you can either install Studio as a standalone application or, if you already have Eclipse installed, you can add Studio as a plugin to your Eclipse installation.
If your work is primarily web application development, we recommend installing the standalone Aptana Studio; if your work also involves Java development, we recommend installing Aptana Studio as an Eclipse plugin.
If you are doing Titanium development, you will want to install either Appcelerator or Titanium Studio.
Is your computer ready? Studio System Requirements
Have an older version of Studio installed? Updating your version of Studio