The Basics
Everyman's Library is a brand or imprint of Alfred A. Knopf, which in turn is a head of the hydra Random House; the idea, as the name implies, is to provide a series of generally affordable editions of what some heap of mewling idiots once saw fit to dub »classics«; most of which, to dodge a long and untopical rant, have nothing to do with the actual Classical period. You probably know the type and whether you like it; if not, we're talking Cervantes, The Brontë Sisters, Henry James, Lampedusa, Sophocles, Tolstoy. That sort of thing. If this still tells you nothing, you're either not the target audience or you have one hell of a ride ahead of you.
The books are bound in cloth of various colors, typically rich, dark tones like burgundy or navy, printed with the title and author of the volume; they are normally provided with dust jackets. The design on these was (I think) recently switched; I personally far prefer the older, cleaner one: black and white, with red frontal lettering. (The other, which I believe to be the new one, involves red and bronze; Deco lines; a pictorial front cover. It may be there are simply two design styles within the series, I honestly don't know.) The color of each book's cover is actually indicative of the time of its writing: if a book is dark green, for instance, this means it is from the 18th Century.
All of the Everyman books I own are, as far as I can recall, set in Bembo, which is a categorically Good Thing.
The Important Bit
These books are quality for your money, serious quality. Understand: there are practically no really good books available in book stores, especially as far as paper quality is concerned, and by my admittedly spoiled standards the Everyman editions constitute cheap, poor-quality books, but they will often be priced at the same level as a large paperback, or even lower. (My copy of Moby-Dick set me back £12 at Foyles; at that same bookshop, the Penguin Classics edition costs £8; the Penguin Deluxe Classics edition, which features a larger format and a badly drawn cover, costs £13.) In contrast, an actually good copy of some reasonably popular older work at a used book store — say, the three-volume set of Rabelais I was eyeing some weeks ago in a Charing Cross Road shop — will set you back at least £50, and probably more.
Consequently, for your money the Everyman editions are superior quality, as they are at least real books, properly bound, unlike the perfect-bound dross which makes up the majority of (new) books sold these days. For (aherm) classics, accept no substitute, except possibly the Library of America, a similar series especially for U.S. writers; the paper in these is thinner and probably of poorer quality, but you will normally get several novels in one volume in an at least semi-decent binding; furthermore, the LA contains many titles absent from the Everyman lineup, such as novels by Philip K. Dick and what I believe to be the complete works of Mark Twain, who is represented in Everyman's Library only by Tom Sawyer and Huckleberry Finn, combined into one volume.
Even if you're low on funds, and the Everyman edition is a bit more expensive, save for a little while or skip your next meal: you'll thank yourself in the future, and you can always curse me in the present.