Online (or "browser-based") eBooks display on your computer screen. You can access them any time you like, as long as you have an internet connection and a library card in good standing.
Once you've logged in with your library card, you will have access to the full text of the eBook, which you can read right there on the screen. You will not, however, be able to transfer the eBooks to a portable device such as a Kobo or Sony eReader.
Many online eBooks can be read on mobile tablets (e.g., iPad, Android tablets), but some require Adobe Flash player to run, and therefore will not work on tablets.