Site Package - a package you choose during installation
Template - a file with tpl filename, it controls the output. witch template to use is controlled by the template override system. Output is mostly (x)html, but it could have been xml or something else if you wanted to.
Website Interface - A site package included in eZ Publish 3.9 and up, it lets you edit content on the front end of your site true a interface, 'front end' is commonly referred to as the website in eZ Publish, hence the name.
Site Style - A specific style/theme for your site using css and images, on ez.no you can find several site style packages that let you install site styles in the admin.
Theme - I don't think eZ Publish uses the name theme anywhere, if it does it is probably referring to Site Style.
