You have a personal (private) account after you registered with your email address and password or when you log in with your Google, LinkedIn, Salesforce or Facebook account. You need a personal (private) account to join or create a team account.
A personal account is however also for business purposes, it merely indicates that it's completely private.
A team account has all the features of a personal account but also offers the option to add your company name, logo and color.
In addition, all team members have access to each other's documents. You can switch between your accounts by clicking in the main menu (upper-right corner).
You can view all plans and features here.