If you have a Kindle, you can't use DRM'd ebooks from any store other than Amazon. (Unless you strip the DRM, which I gather is not difficult, but a lot of people are strongly invested in convincing you that's illegal. It is, at the least, a hassle.)
Any non-Kindle reader can't use DRM'd ebooks from Amazon--and possibly can't use any books from Amazon unless they're converted first.
The Kindle reads: .mobi/.prc ebooks, either non-DRM'd, or with Amazon's DRM .tpz, Amazon's "topaz" format, only available through them (and nobody likes it) .pdf ebooks, without DRM (badly. None of them are good at PDFs) .txt files, which are almost useless as ebooks.
The Nook reads .epub & .pdf with B&N's DRM, other Adobe Digital Editions (ADE) DRM (there are two types), and non-DRM'd
Sony reads: .epub & .pdf with ADE, and without DRM .txt .rtf
Kobo reads: .epub & .pdf with ADE non-DRM'd .mobi (so, files from Amazon *that don't have DRM*, or have had it removed, would be readable) .txt, .html, .rtf, .cbz, .cbr (however, I have no idea how good the support is for these; I have a reader that claims to read these and does it horribly.)
None of the other readers currently strongly marketed read DRM'd .mobi ebooks; the industry is shifting to .epub. (It's a much more versatile format; .mobi is a holdover from the PalmOS days.)
Waterstones is tied to Sony; they sell in .epub, and I believe it's all DRM'd. Blackwell also uses Adobe Digital Editions; it's unclear whether everything is DRM'd, but you can probably expect it is.
A lot of ebookstores try not to tell you exactly what filetypes they sell; several don't tell you whether the files are DRM'd or not. (B&N sells some with & some without--but refuses to tell you which are which. Amazon's can only be identified by the device # restriction on the book listings. (If it says unlimited devices, it doesn't have DRM.)
It's probably easiest to get a Sony, buy from Waterstones & Blackwell directly for it, and buy from Amazon through the PC program and use another program to strip the DRM, then use Calibre to convert to .epub. But only probably. And stripping the DRM may or may not be legal in your area. (Nobody has ever been prosecuted, nor is likely to be, for removing DRM on their own purchases.)
That's a lot of info to throw at you at once; I'm happy to go over more if you'd like, but I don't want to overwhelm you or swamp you with details that won't matter to you.
Re: Basics: What can and can't I do with readers?
Any non-Kindle reader can't use DRM'd ebooks from Amazon--and possibly can't use any books from Amazon unless they're converted first.
The Kindle reads:
.mobi/.prc ebooks, either non-DRM'd, or with Amazon's DRM
.tpz, Amazon's "topaz" format, only available through them (and nobody likes it)
.pdf ebooks, without DRM (badly. None of them are good at PDFs)
.txt files, which are almost useless as ebooks.
The Nook reads
.epub & .pdf with B&N's DRM, other Adobe Digital Editions (ADE) DRM (there are two types), and non-DRM'd
Sony reads:
.epub & .pdf with ADE, and without DRM
.txt
.rtf
Kobo reads:
.epub & .pdf with ADE
non-DRM'd .mobi (so, files from Amazon *that don't have DRM*, or have had it removed, would be readable)
.txt, .html, .rtf, .cbz, .cbr (however, I have no idea how good the support is for these; I have a reader that claims to read these and does it horribly.)
None of the other readers currently strongly marketed read DRM'd .mobi ebooks; the industry is shifting to .epub. (It's a much more versatile format; .mobi is a holdover from the PalmOS days.)
Waterstones is tied to Sony; they sell in .epub, and I believe it's all DRM'd. Blackwell also uses Adobe Digital Editions; it's unclear whether everything is DRM'd, but you can probably expect it is.
A lot of ebookstores try not to tell you exactly what filetypes they sell; several don't tell you whether the files are DRM'd or not. (B&N sells some with & some without--but refuses to tell you which are which. Amazon's can only be identified by the device # restriction on the book listings. (If it says unlimited devices, it doesn't have DRM.)
It's probably easiest to get a Sony, buy from Waterstones & Blackwell directly for it, and buy from Amazon through the PC program and use another program to strip the DRM, then use Calibre to convert to .epub. But only probably. And stripping the DRM may or may not be legal in your area. (Nobody has ever been prosecuted, nor is likely to be, for removing DRM on their own purchases.)
That's a lot of info to throw at you at once; I'm happy to go over more if you'd like, but I don't want to overwhelm you or swamp you with details that won't matter to you.