Samsung Captivate (Galaxy S)

Service providers normally block root access so they can control your use of the phone. For example, to keep you from deleting the bloatware in case you change your mind later, but more likely so that you might click it by accident, sign up for some extra service by accident (like navigation service), and then forget to cancel that extra service. They also block root access to keep you from side-loading appd, which is to load an application that's not from the Android market. In additiion they filter what you can get from the marketplace, so that you can't get an application to allow tethering (but the Airplane mode/WIFI gets around that very easily). Tethering lets you share the internet connection on your cell phone to some other device, like a laptop. Providers don't like this because it's easier to use up large amounts of data on a laptop than a phone.

Back to your queston, "rooting" refers to opening up the ability to get to the "root" level of the operating system of the phone. At the level you can do anything you want to the phone, including loading any apps from any source and removing the bloatware. But there are some risks associated with it, including possible warranty issues, and "bricking" the phone (rendering it unusable to the point you can't even get back to your original state of operation). For the Captivate, rooting itself is fairly low-risk. The "bricking" risk comes in if you're trying reload the operating system using a ROM file that someone else has put together ("cooked"), usually found on a website (it may be a bad ROM, or just the wrong one for your phone). But if you're just rooting to allow side-loading and deleting bloatware, they you're pretty safe.

But should you do it? Even though it's fairly easy, I have not done it on mine because I can just ignore the bloatware and I can find all the apps I've ever wanted on the Android market (especially when using the Airplane mode/WIFI technique to see everything). So unless you have a compelling reason why you think you need to do it, I wouldn't mess with it.