Glossary
Routing and Slugs
The rules by which TYPO3 builds addresses and resolves them back to content.
From path to content
Routing answers two questions: which address a piece of content gets, and which content belongs to a requested address. Both are settled in the site configuration.
For ordinary pages the slug is enough. It gets more interesting with extensions that output records of their own: without configuration, an event detail page would be an address with parameters appended. So-called enhancers move those parameters into the path, turning a technical address into a readable one.
Why the structure should be settled early
Every later change to the address structure drags redirects behind it. That is manageable, but it adds up: rebuild three times and you have three generations of old addresses to serve.
So the question of what addresses should look like belongs at the start of a project, together with the language structure, because the two interlock. There is more on this under TYPO3 SEO.
Several languages
With more than one language, the configuration decides whether a language runs behind a prefix in the path or on a domain of its own. Both are legitimate; what matters is that every language version has an unambiguous address and that the links between them are correct.