JavaScript is mostly harmless and style sheets are pretty darn nifty. Enable AutoInstall tells the browser whether you want it to be able to download and install plug-ins and other updates. Call me crazy, but I'd rather be in complete control of downloading and installing software on my machine. I'm no control freak, but if something goes wrong, at least I'll know who to blame. Briefly, a cookie is a small message to itself that a web site leaves on your hard disk.
Some Internet users see cookies as a threat to their privacy.
If you're more trusting, you can accept all cookies. If you accept cookies, you can ask Navigator to warn you before accepting each one. This will become so tedious that you'll surely turn that feature right back off again. All those settings, and there's still no check box that will prevent web sites from sending and playing those annoying plinky-plunky MIDI songs. The cache is where Navigator temporarily stores copies of the web pages that you've visited recently.