Hello tibiafry and welcome. there are a lot of very knowledgeable and helpful people here who can give you so much information you will soon be drowning in it.
probably to best help you it would be useful to know more about your reasons for learning about ebooks (personal use ? publishing ? archive ? reference ?).
to give you a first answer, the new industry standard format is epub and i recommend anyone starting out in ebooks turn in that direction. it is an open format based on xhtml and css, already very robust and complete, and is still being defined, so will continue to improve, as will the viewers for epub files. it can be used as a source or end format (meaning, you can use it to archive your files, it can easily be converted to other popular formats, and it can also be read natively on more and more devices : the sony 505 and 700, iphone using stanza, and any browser). i recommend you read at least the first page of
this thread about epub.
as for drm, if you mean to create drm books, you should know that the overwhelming majority of ebook readers agree that it is preferable to avoid it ; it does not actually securise any files, and will do nothing to prevent copyright infringement, however it does cause significant inconvenience and prejudice to legitimate, paying customers, and in fact in some cases it can be argued that it actually encourages piracy. just as the music industry has finally caught on and is abandoning drm, so the publishing industry will in the end stop using it too i'm sure (hopefully sooner than later). if you are asking about which kind of drm files to buy, many people buy Lit format (microsoft windows) books, because the drm can easily be removed, and the book can then be converted to another format for your personal use.
take a look around the forums and the wiki (link at the top left of the page) ; i'm sure you will find a lot of information here ! and don't hesitate to ask questions.