When you install NoScript, JavaScript, Java, Flash Silverlight and possibly other executable contents are blocked by default. You will be able to allow JavaScript/Java/... execution (scripts from now on) selectively, on the sites you trust. You can allow a site to run scripts temporarily, if you're just surfing randomly, or permanently, when you visit it often and you really trust it. This means that NoScript learns from your own browser habits and tends to disappear in the background after a while, but it promptly comes back to save your day if you stumble upon a malicious web page.

When you browse a site containing blocked scripts a notification, similar to those issued by popup blocker, is shown.
Look at it or at the statusbar icon to know current NoScript permissions:

* Forbidden Icon - this means that scripts and plugin contents are blocked for the current site and its subframes. Even if some of the 3rd party script sources imported by the page may be in your whitelist, no code could run because the hosting documents are not enabled.
* Partially Allowed Subcontent Icon - this means the top level site is still forbidden but some active subcontent pieces (either frames or plugin objects) are allowed: some code may be running, but the page is likely not to work correctly yet because its main script source is still blocked.
* Partially Allowed Icon - this means scripts are allowed for the top-level (main) document, but some other active content or script sources imported by this page are not allowed yet. This happens when there are multiple frames, or script elements linking code hosted on 3rd party hosts.
Since they're often unnecessary, the site is likely to work even in this "partially allowed" state. Furthermore, in most cases when a site is compromised with JavaScript malware, the malicious code is hosted on external "shady" sites. Even if you've previously allowed the top-level site, these external sites are still blocked and the attack fails anyway.
* Partially Allowed / Partially Untrusted Icon - this means scripts are allowed for some URLs, and all the other ones are marked as untrusted.
* Allowed Icon - this means that script execution is allowed for the current site
* Globally Allowed Icon - this means that scripts are globally allowed (why did you decide to browse without any protection??!)