The User Login Form block is a block you should select when you want to put a login section to your web page. Hover above the section where you want to add this, click on or and select User Login Form:
Select the relevant button depending on where you want to see this block: Show on every page, Show on the current page only, or Share across many pages.
Add the Block heading, check the Published box, select from Block sizing options, choose Page Option from the drop-down menu and click on Save component when you are finished.
Your User Login Form will show on your web page, though please note that it will look different on your web page depending on the Page Option selected.
When you select User Login Page, you will have two sections: one for already registered customers and one for new ones:
If you select Payment Login Page, this will provide your customers with an option to purchase items from your website without an account, to register or to sign in if they already have an account: