Extensionless URLs omit the web address extension (html, asp, aspx, php, jsp, .cfm, etc.). With Extensionless URLs, a web address is independent of the underlying technology.
For example, this page uses php - but the web address you will see above is
If this page's underlying technology were changed to .aspx or .jsp, the web address would remain constant. External and internal links to this page would be preserved, without redirects. There are many pages on the Internet where the purpose and function has remained the same for a decade or more, but the underlying technology may have changed from hand-coded HTML to ASP to Cold Fusion to Word Press - all with different extensions. An extensionless URL can keep the address the same!
Extension-less URLs: Aesthetics, Functionality, Image
Extensions are a content-free mechanical item that can be concealed, like an engine under a hood.
Extensionless URLs are easy for humans to remember:
is much better on a billboard, bus sign, ad display at a sports venue than
The second example ("offer.aspx") tends to focus the mind on the extension, not the company name and web address. This detracts from branding.
Large companies with sophisticated sites increasingly use Extensionless URLs; using extension-less URLs is a subtle way of demonstrating technical excellence. Old, unrevised websites generally have extensions. Extensionless URLs look clean and modern.