There are basically two types of user who could access WHM either you are reseller or you are Root.
In case of Reseller you get WHM to host multiples of cPanel accounts. You can create multiples cPanel account as per the plans you have purchased. In WHM(Reseller) you get limited powers as compared to WHM(Root).
On the other hand if you have access to WHM(root) then one thing you need to understand that you are the owner of the server and can manage things as an administrator with ultimate power. You get the authority to provide shared hosting, reseller hosting, VPS to end users. You also get many other features with WHM like installiation/Uninstallation of softwares and adding plugins as per your need. You can also create Multiples of Reseller and Master Reseller accounts as per norms.
Whereas cPanel is your website’s control panel, and allows you to do tasks such as edit files, create email addresses, and setup databases. The cPanel accounts themselves are managed by Web Host Manager, commonly referred to as WHM. WHM access is not available in Shared Hosting.
Hence we can say that Web Host Manager, or WHM, is a powerful interface that allows administrative access to the back end of cPanel.
What are the different ways to open WHM? [Updated May 2019]
There are various ways to open WHM. You can log into WHM using variations of your domain name running on the same server.
The following is a list of various URLs you can use to log into your WHM: [Important 2019]