URL structure helps a lot to get your website ranked. Even Google instruct webmasters to give attention to the structure of URL. A good URL always get a better chance to win the ranking game.

What is a good URL structure?

A url that is simple and descriptive is considered good. A good URL create a context and improves the chance of being chosen by the users. A url should be made of readable words rather than numbers, IDs and other non-readable parameters.

Hyphen or Underscore?

You can use the hyphen (-) in the URL but avoid the underscore ( _ ).

A site’s URL structure should be as simple as possible. Consider organizing your content so that URLs are constructed logically and in a manner that is most intelligible to humans. When possible, use readable words rather than long ID numbers in your URLs.

Source: Google’s URL Structure Guideline

Human Readable URL Structure

The URL should be human-readable. A user can know the content of the page by reading the URL.

Examples of Good and Bad URL structures

Correct	example.com/learn-search-engine-optimization.html
Incorrect	example.com/learn_search_engine_optimization.html
Correct	example.com/learn-search-engine-optimization.html
Incorrect	example.com/12345.html
Correct	example.com/learn-search-engine-optimization.html
Incorrect	example.com/?pid=12345
Correct	example.com/learn-seo/filter?city-location=california
Incorrect	example.com/learn_seo/filter?city_location=california

The URL should be enough descriptive to make users click on it even after ranking in the bottom ranks. Higher CTR is an indicator of good URLs.