For maximum readability and SEO use kebab-case (dashes) in your URLs and make them short and friendly.
Learn more on Best Practices for URL Structure.
northwind.com/pageonworddocumentation
Figure: Bad example - No kebab-case in URL
northwind.com/PageOnWordDocumentation
Figure: Bad example - PascalCase (better readability and still works in small caps, but other people might share it without the MixedCase)
northwind.com/page on word documentation
...will become
northwind.com/page20%on20%word20%documentation
Figure: Bad example - spaces it will show up in your URL structure as 20%, which is bad for readability and SEO
northwind.com/pageonword_documentation
Figure: OK example - underscored (snake_case) URLs have good readability but are not recommended by Google
northwind.com/page-on-word-documentation
Figure: Good example - kebab-case is recommended by Google. Note: Don't use any uppercase
Not for domains! This is only for pages and documents. Domains are bad when they have dashes in them.
Read more on SEO 101: Hyphens vs. Underscores in URLs.
More info: You can install the IIS URL Rewrite Module for IIS7 you can make ugly URLs much more friendly.
The caveat here is that it will only work if the URL is in the clear on the page.
Note: This could only be done with certain links as others are postbacks as well.
