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 work with other users, and add your company name, logo and color.
It is important to note that team members who have been invited to a team must be logged in under the Team they have been invited to in order to have access to the paid features of the plan. It is possible to set a team as your default account from the Account page.
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.