A domain name is a unique web address on the Internet, like www.example.com. Much like buying real estate in the physical world, when you register a domain name it becomes your property in the online world. No two domain names can be the same which means that if the domain name you want is already registered, you are unfortunately out of luck. Domain names can also have different domain extensions, or TLDs, attached to the end of it. For example, you may own www.example.com, however www.example.net may still be available.
Many services offer so-called unlimited or unmetered service for whatever amount of bandwidth, disk storage and sites you use. It's important to understand that most terms of service actually do limit the definition of "unlimited" to what's considered reasonable use. The bottom line is simple: if you're building a pretty basic website, unlimited means you don't need to worry. But if you're trying to do something excessive (or illegal, immoral or fattening), the fine print in the terms of service will trigger, and you'll either be asked to spend more or go elsewhere.