Recs.
Updated
Mobile Web apps are accessed via the smartphone's browser, and use techniques such as media queries to customize the styling for the various mobile phones and tablets.
Specs
Pros
Pro Faster and cheaper to develop
The biggest benefit of developing mobile web apps is that they are significantly easier, faster and cheaper to develop then native applications.
It is a lot easier to find (and cheaper to hire) developers familiar with web technologies compared to those with experience with a particular mobile platform. The same trend can be seen with hiring designers, as they are most likely already familiar with HTML & CSS.
Another factor to consider is that web apps can share a single codebase with the desktop version by using media queries to switch CSS stylesheets for mobile devices. This can help reduce the technical debt on developers.
Pro You can use the web for distribution
Mobile web applications can use your existing website as an effective distribution channel. Users can be automatically be redirected to a mobile friendly version of the site with no installation required.
This has the added benefit of an improved on-boarding experience compared to native apps as shown in the image (source).
Distribution via the web has the added advantage of being able to constantly push updates to the app without waiting on both approval by the app store and for users to install the update. This is especially handy for quickly fixing bugs and rapidly iterating.