In Ractive, templates (until they get parsed, at least) are just snippets of HTML, with a few differences. A template should be well-formed - Ractive's parser is not quite as forgiving as the HTML parsers found in browsers (though it does allow things like implicitly closed elements).
Strictly speaking, a Ractive template is not valid HTML (for one thing, 'valid HTML' describes an entire document, and we're only dealing with snippets), but it doesn't matter, even if you're the kind of obsessive who can't stand a single error in the W3C validator, because it all comes out as lovely standards-compliant DOM.
For reference, however, the differences between Ractive templates and HTML are listed on this page.
The most obvious difference is that Ractive templates contain mustaches to facilitate data binding.
Elements in a Ractive template can have events, which look like attributes but don't get rendered to the DOM as attributes (they are intercepted, and used as event binding instructions instead):
Another item in the set of things-that-look-like-attributes-but-aren't, transitions allow you to specify how elements should behave when they first get introduced to the DOM and when they get removed from it later:
<div fade-in>This element will fade in gradually when it renders</div>
Decorators can decorate elements as they are rendered to the DOM. These also look like attributes, but they never get added to the DOM themselves.
<div as-ace-editor>This element will be turned into an Ace Editor.</div>