Ebooks are books that have been digitized, whether in pdf, html, lit, chm or other file formats.
They can be cumbersome to read on computer monitors, which is part of the reason many can be found for free on the net. Publishers think of it as good marketing, giving you a quick taste and then you pining for a hardcopy of it. Kind of like how drug pushers hook kids.
There are many people who actually read the ebooks they download, and there are many who just collect them addictively, never reading them, only hoarding them.
Finding ebooks, not an easy task to the uninitiated, but everyone gets the hang of it, in time.