It does, in some poorly-designed Web clients. Definition lists do not contain items, but rather terms and definitions. Play with your own document, etc. You get the gist. To keep them on the same line if possible, use the compact attribute.
Attributes go inside the start-tag for an element.
You know what to do. They generally cause line-breaks and have some space before and after them. Ninety percent of your audience will not notice the difference. This is a paragraph. Note that all of these elements require end-tags.