Website hosting is typically measured in the amount of disk space you're allotted on the server and the amount of data transfer or "bandwidth" you need for accessing the server. For example, if you have a lot of customer interaction at your website, such as files to download, you will access the server frequently and you'll need more Web hosting transfer space than someone who simply puts readable text on their website. The more "items" or "content" you have on your site (i.e., photos, maps, PDF files, etc.), the more disk space you'll need for website hosting.
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.
For example, only organizations can register a “.org” domain name, and only American citizens can register a domain name that ends in “.us.” Failing to meet the guidelines and requirements for each type of domain during the actual registration and payment process will result in the domain name being “released” back into the pool of available domain names; the customer will have to pick a top level domain for which they actually qualify, or cancel their purchase altogether.
In the first quarter of 2015, 294 million domain names had been registered. A large fraction of them are in the com TLD, which as of December 21, 2014, had 115.6 million domain names, including 11.9 million online business and e-commerce sites, 4.3 million entertainment sites, 3.1 million finance related sites, and 1.8 million sports sites. As of July 2012 the com TLD had more registrations than all of the ccTLDs combined.
Migration or transfer services are often free or offered at a reasonable fee. These services help move your existing site to the new hosting provider. They can save a huge amount of hassle. Just remember that the migration process is often automated, and may fit in with the host's processes and needs rather than yours. Not everything may migrate, and you may find the organization of the newly migrated site makes for harder maintenance in the long run.