Tumblr offers the option to point a custom domain name to your blog. This allows professional users to cultivate their website on Tumblr without sharing the dubiously professional "tumblr.com" subdomain, though personal users may use a custom domain for fun. Tumblr doesn't sell domains, so you need to own your domain through a registrar before setting it up on Tumblr. You also must change the A-record settings before setting up your custom domain name.
Setting Up GoDaddy for Tumblr
To use your GoDaddy domain with Tumblr, you'll need to change the DNS settings to point to Tumblr. Open up the GoDaddy account manager and click "Launch" by domains. Choose "Edit Zone" next to your domain, then click the "@" symbol under the "A (Host)" section. Enter "22.214.171.124" in the "Points To" section and save the zone file. Note that it can take 48 to 72 hours for DNS settings to update.
Setting Up a Custom Domain on Tumblr
With your GoDaddy URL set up, go to Tumblr and click the settings icon resembling a gear. Select the blog you want to set with your domain and click the check box beside "Use a Custom Domain Name" in the URL section. Enter your domain as "yourdomain.com" -- you do not need the "http://" or "www." Click "Test Domain" to confirm that the domain works. Save your settings when you're done.
If you've used your Tumblr blog for a while, you undoubtedly have backlinks across the Internet that use your "username.tumblr.com" URL. These links still work after you've set up a custom domain name. When you set up a custom domain name, all traffic that points to "username.tumblr.com" automatically redirects, updating the links to replace the old Tumblr sub-domain URL with your custom domain name.
Interacting on Tumblr
When you interact with other users on Tumblr, they see your username, not your domain name. So, if someone reblogs one of your posts, it will show your username as the link within the post. This allows users to find your blog even if they don't know your domain, and to know who they're interacting with it.