What is an email client?
An email client is software that you download and install on your computer, and use to send and receive emails. The alternative is to use webmail.
All email travels over the internet, and is stored on servers. Those servers can belong to email providers (like Gmail), internet service providers (like Comcast), or web hosting providers (like GoDaddy). This server is where email is collected and stored, until you delete it.
An email client must be connected to the internet to send and receive email, and is configured to access your provider's email servers. Once your email is downloaded to your computer from your provider's server, you can use the email client to read and interact with your email, even if you're not online.
Email clients offer more robust functionality than a webmail website. However, the client must be configured correctly, and on each device separately, to work with your email provider's servers. Office 365 allows you to configure many clients, such as Outlook and Apple Mail. You can also use the native email clients, or the Outlook app on your iPhone, Android, or Windows mobile device.