While the the best of them offer surprising amounts of flexibility, they also impose stringent enough restrictions to page design that you shouldn't be able to create a really bad looking site using one of these services. Typically you can get a Mysite.servicename.com style-url with no commerce abilities for free from one of these services; you have to pay extra for a better URL and the ability to sell. One issue to consider is that if you eventually outgrow one of these services, it can be hard to export your site to a full scale advanced web hosting like Dreamhost or Hostgator. If you know that's where you are eventually going, it may be better to skip the sitebuilder step.
A website is a collection of many types of content. Your website can contain a collection of content including blog posts, product descriptions, images that highlight your product, explainer videos that tell your story, and contact information. These different types of content have one thing in common. Each piece of content has seconds to capture the attention of your audience.