Products can have several different variants and they can be represented in several different ways. An example of variants you may use are size/colour/style, etc.
To add a variant to your product, edit the product and check the 'This product has variants' box:
To add a new variant to your product, click the 'add variant' button, which will open up a new window where you can enter details for the new variant. You can enter the name of variant, the type of variant (see below for further details), whether it's required or not and then you can add multiple options for certain variant types:
There are four types of variants you can add to your products:
This will display as a dropdown list on the product details page, enabling the user to select from a predetermined list of options.
This variant type also has predetermined options, but it displays all options and only enables the user to choose one.
This is a single line of user-entered text. It could be used to enter order customisation info (e.g. a name or number) or it could be used to enter a date or other info.
This is similar to the single line variant, but it's a text box where users can enter multiple lines of text.