The web host saves your website files, posts, videos, images and keep them safely, it provides branded email addresses (like email@example.com) and many other services.
These days Most companies are similar as they offer almost same features, it just comes down to cost and who you’re comfortable with.
So after you have registered your domain name you should subscribe to a web hosting service to have a place to host your site.
You can subscribe for a month/s or for a year/s (the cost will varies depending on the period).
For an affiliate marketing website and as a start it is recommended to register in “shared” hosting as you can then save money.
Shared account is enough for small and medium websites, and when your website becomes big (as with over than 50000 visitors a day) then you can subscribe in “dedicated” hosting plan which provides a private server for you (while shared plan provides same server for few websites, (but don´t worry as your website will not affect at all).
See my article: Best Web Hosting Providers and decide which one is better for you.
When you make a decision and choose a provider go to: Sign up and Register a domain Name in HostGator to learn how to sign up web hosting for your domain name.