What's the difference between my Developer account and a "Company"?

When you first register with the site, you're creating a personal developer account.

If you're working on behalf of a company or client, we recommend you set up a
"Company" in the site (go to your account drop down and select Manage Companies).

A "Company" account lets you set up a separate billing and subscriptions for that company or client. You can also invite other developers to use API keys associated with that account while maintaining the security of your own separate personal developer account.