You could think of the sites that share your server as your roommates; there's really not that much separating you from them. Sure, you can close the bedroom door, but they can still cause nightmares for you in the kitchen and the bathroom. In web hosting terms, all the sites share a single server's resources, so huge traffic spike on Site A may impact the neighboring sites' performances. It's even possible that another site could take down the shared server altogether, if it crashed hard enough.
When you build a website, you want visitors to come and see what you've done. To get them there, you need a unique domain name that connects to your sites servers. Domain name registration is required to ensure that no one else in the world can claim ownership of your web site's address and to make finding your website simple. Find your one of a kind domain name.
This way, you really never have “your own” domain. The name of your free website builder provider is going to be permanently put in the website’s URL. Surely, some personalization is still possible – as you get to choose the name of the project but that’s really where it ends. This little issue actually makes perfect sense. After all, it would cost additional money for your provider to register a full custom domain name just for you. So if you want your page to spell “yourwebsite.com”, a free builder won’t suffice.
VPS plans are similar to shared plans in that both feature multiple websites hosted on a single server. However, VPS plans maintain a strict separation between clients and websites when it comes to resource allocations. Your site gets its share, and no one else may use your resources (conversely, you may not cannibalize the resources allocated to others' websites either).
Many services offer a low "starting price," but require you to prepay for two or three years of service to get that price. After the promotional period, the renewal price for some services can be two, three, or even four times the initial promotional pricing. While the initial deal might be incredible, the cost of transferring your site (or paying the added fee) in a couple of years may be something to consider.