A native app is developed for a specific platform (e.g. Android or iOS). As a user, you should always download this app from the Google Play Store or the AppStore. After installation, you can use this app on your smartphone or tablet.
A web app is basically a mobile version of a website. The website will then be built ‘responsive’, which means that it will also be displayed well on other devices than just the PC or laptop. The content of the website then scales with the size of the screen. A web app does not need to be downloaded, but can be accessed via the internet browser.
A hybrid app is basically a web app, but can use a number of functions of your smartphone.