Where to store plugin settings in the database is technically up to the plugin developer. I’ve seen some plugins that create custom database tables.
The standardized method, however, is to use the WordPress site options table (i.e. wp_options). That’s what will be used if the developer uses the WordPress settings API.
You can view a list of all of the saved options if you visit http://yoursite.com/wp-admin/options.php.
Many plugin developers don’t do a single setting per database field, though. Instead, they save an array of settings into the database field. That means that if you try to view them on the options.php page, you’ll just see SERIALIZED DATA in the field. If you try to view the field in something like phpMyAdmin, you’ll see a bunch of curly braces, colons, and numbers that are organizing the data. If you plan to modify them directly in the database, I’d recommend you do some research in to PHP data serialization before doing so.