A domain is your identity online. It separates you from everyone else - once yours, as long as you keep renewing it, no one else may have it. 

There are two parts to a domain - the domain name itself, and the TLD (top level domain) extension. 

The most popular TLD is the .com extension - it is synonymous with online business. But due to its popularity, getting a good domain name with '.com' is hard - most have been bought and are not available. 

Most TLDs have a meaning behind them - .EDUs are used by educational institutes, and .ORGs are supposed to be used by non-profit organizations. A domain with a TLD of 'co.uk' is very likely British-based. There are other TLDs too, such as .cc, .tv, and .tk. These are all associated with either a company or with a country. 

For most domains, the minimum registration duration is one year. Some TLDs such as .US require a minimum of two years. However, most TLDs only require a one year commitment. 

Once you have registered a domain, as long as you remember to keep renewing it, no one will be able to take the domain. You may sell the domain to someone else (or even buy somebody else's domain), but no one will be able to just take the domain. 

Having your own domain is very important for your business. It sets you apart from everybody else, and gives you a brand to work with.