A Web App is simply a website (uses the same technology as a computer website: HTML, CSS, JS) optimized for mobile devices (phones/tablets) that look like a Native App, using a Responsive Layout for providing an optimal experience for all mobile devices and orientations (landscape/portrait).
Responsive Layout = Fluid Layout (width %) + Flexible Image (width %) + Adaptative Layout (Breakpoints)
Design and Front-end by danielmrey: This Wesbite > HTML + CSS + JS + PHP
Design and Front-end by danielmrey: Prototype > Jquery Mobile
Design by Mercer Design, Front-end by danielmrey: Presentation > Sencha Touch
Design and Front-end and programming by danielmrey: Fun > HTML + CSS + JS + PHP
Design and Front-end by danielmrey: Template > HTML + CSS
Design by Crystal Interactive, Front-end by danielmrey: Events > HTML + CSS + JS
I created many Web Apps for the event industry when I was working in Crystal Interactive but I cannot show them here due to copyrights.