Spotlight is a fantastically powerful tool included on all Macs as part of Mac OS X. When explaining what Spotlight is to people, I liken it to Google for your Mac. Google is great for searching the web for just about anything. Spotlight is great for searching your Mac for just about anything, including calendar events, contacts, music, movies, etc. You can access Spotlight either by clicking on the magnifying glass in the top right-hand corner, or by pressing Command+Space Bar.
Relying on Spotlight too much can actually make it difficult to remember where you save your files. Sure, it's quick and easy to access them, but sometimes you need to know the location of the file itself. Simply searching for a file using Spotlight won't give you that path... or will it?
Here's a real world example. I downloaded a file from Adobe yesterday. In order to download the file, I was required to install the Akamai downloader tool. Fair enough. The problem is that when I was done using it, I wanted to remove it, but it wasn't obvious where it stored that application.
After looking in my system Applications folder and not seeing it, I performed a quick Spotlight search and found the file. Next, I needed to know the path of where that file lived so that I could delete it. To do that, I highlight the file in Spotlight, then press the Command key. When you do this, you will notice that the path of that selected file will appear at the bottom of the Spotlight window. When you let go of the Command key, the path will disappear.
Here's a video showing you what it looks like: