By definition, a domain name is simply a human readable form of an IP address. In function it is the destination that you type into a web browser in order to visit a website, such a www.google.com. Metaphorically, it is very similar to how you would scroll to a contact in your cell phone rather than manually dialing the person by entering their full phone number; the phone number would be an IP address and the saved contact would be a domain name. Always buy a domain with a reputable domain registrar.
The host may also provide an interface or control panel for managing the Web server and installing scripts, as well as other modules and service applications like e-mail. A web server that does not use a control panel for managing the hosting account, is often referred to as a "headless" server. Some hosts specialize in certain software or services (e.g. e-commerce, blogs, etc.).
The availability of a website is measured by the percentage of a year in which the website is publicly accessible and reachable via the Internet. This is different from measuring the uptime of a system. Uptime refers to the system itself being online. Uptime does not take into account being able to reach it as in the event of a network outage. A hosting provider's Service Level Agreement (SLA) may include a certain amount of scheduled downtime per year in order to perform maintenance on the systems. This scheduled downtime is often excluded from the SLA timeframe, and needs to be subtracted from the Total Time when availability is calculated. Depending on the wording of an SLA, if the availability of a system drops below that in the signed SLA, a hosting provider often will provide a partial refund for time lost. How downtime is determined changes from provider to provider, therefore reading the SLA is imperative. Not all providers release uptime statistics. Most hosting providers will guarantee at least 99.9% uptime which will allow for 43m of downtime per month, or 8h 45m of downtime per year.