The Semantic Views module has been mostly incorporated into Views 3.x. Semantic Views is still around for people who need it, though. For some details on how the original module is different from the Views implementation, please see this issue.
Semantic views help you insert markup of your own from the Views UI, so that you can fairly easily override the default markup without having to restyle via templates.
As a usage example,
In a view with a field:
Configure the field. (Click on the field.)
In the modal that opens, scroll down to Style Settings.
Choose one or more of the three Customize options. This will reveal a dropdown menu where you can choose from one or more HTML tags to use on that field and allow you to add a CSS class specific to that field should you desire.
Decide if you want to keep the Views default classes. Unchecking this box means your markup is *it*.