Empty pages

Books have been traditionally bound in two styles for about the last five hundred years--quatro and octavo. Quatro means that the sheet has been folded twice, so that when bound and cut, that sheet produces four sheets comprising 8 pages. Octavo means the sheet is folded three times, so that when bound and cut, the sheet produces eight sheets, comprising 16 pages. It is simpler and more cost effective to print the sheets, fold, bind and cut them, leaving blank sheets, than to try to get too fancy with the process. This is likely more the case in the modern world, as most of this process is now done by machine. I can't cite a source for you on this, but my grandfather, who raised me, was a printer by avocation, and made quite a good bit of money on the side printing special orders, and bank and school stationary--i used to catch hell because he printed the report cards.

Setanta is right.
Take a look at your newspaper. The number of pages will be a multiple of 8 (if tabloid) or a multiple of 4 (if standard).
If that isn't exactly the case -which happens rarely-, then a "tripe" (a special cut paper) was used, at the expense of the publisher or, more likely, at the expense of some announcer who made a last hour request for publicity.

Yep, that's right.

Another note, if you're looking up a used book and it mentions 8vo or 4vo... that is referring to octavo and quatro... not number of volumes. An 8vo is a smaller size book. 4vo... is incorrect and should be 4to but a lot of book sellers make this mistake!

And here's probably more than you wanted to know:

Quarto (sometimes abbreviated to "4to"), a term from bibliography, refers to the format of a book. A quarto is the second largest format, coming in between the folio and the octavo. A sheet is folded over twice, so each sheet consists of four leaves (or eight pages). A typical quarto is a bit larger than the average hardcover book today.

Just as the quarto falls between the folio and the octavo in size, it falls between them in dignity. The most serious materials (such as Bibles) have traditionally appeared in folios, whereas more popular works (such as novels) appeared in octavos.

They're useful for making notes and what not!

<nods>

simple and clear explanation, thus.

thank you all very much for the interesting details.

Groups of pages . . .
The groups of pages are called "signatures" - as stated before, they can be anything from 4, 8, 16, 32, 64. I'm not aware of 128-page signatures, but I won't say there aren't any. The placement of which page prints where is called "imposition" . . . that is, which page backs up which page in the printing process. Multiple pages are locked up in a form to be printed on one sheet of paper. This is for sheet-fed "letterpresses." And on, and on, and on. Rotary presses, like newspapers that print off monstrous rolls of paper (3,000 lbs.), is another matter. Look down the backbone of a book where the gap is and you can see the signatures and how many pages they are composed of. Most often, they are all uniform. However, there might be a "tip in." Anyway, if you write a 254-page book, it will be printed on 256 pages, eight 32-page signatures (usually not sixteen 16's, etc.). Sixty-four page impositions are usually used for larger books.