Not exactly:

"The script editor can be accessed directly at http://script.google.com, or by launching it from one of the Google products which support built-in access to the script editor, such as Google Spreadsheets and Google Sites."

In Spreadsheets, you just need to go to Tools → Scripts → Script Editor.

Not a separate app, in any meaningful sense of the word. I use it extensively for personal use; I have a couple spreadsheets online involving personal data and I just click a button in the menu for spreadsheets to open up the script editor.