Attribution note: this page was derived from this wikipedia article).
The cards are dealt, face up, into eight columns (or piles) of six cards each. (These eight columns make up the tableau.) The remaining four cards go into the reserve. When dealt, the table should look somewhat like this:
r f f f f r r c c c c c c c c r c c c c c c c c _ c c c c c c c c _ c c c c c c c c _ c c c c c c c c _ c c c c c c c c
The eight slots on the left-hand side of the figure represent the reserve. The reserve can be used to temporarily store any (top) card from the table. Four of the reserve slots (sometimes called 'cells') are filled at the beginning of the game (these four slots are represented by the 'r's in the figure; the empty slots are represented by underscores).
The four 'f's at the top of the figure represent the four foundations. These, as in Klondike, are meant to be built up in suit from Ace to King. That is, each foundation begins with the Ace of one suit and is followed by the 2 of the same suit, which is followed by the 3 of the same suit, and so forth, until all the cards through the King have been placed on the foundation.
The 'c's that fill the majority of the figure represent the columns where the game play actually occurs. The cards are, again, all face up, and are built down, traditionally by suit. (Players can modify the difficulty of the game, if they like, by building down in a different manner. For example, one could play by alternating colours, the way Klondike is played.)
Technically, one may only move the cards between columns one at a time; however, the presence of a free reserve slot essentially increases the number of cards that can be moved. (e.g., if there are three open reserve slots, four cards can actually be moved at once -- one for each reserve slot, and the one that can always be moved.)
If a column is emptied, most rules allow for one to place any card in the empty space, regardless of suit or rank (as long as it follows the other restrictions on moving cards). Advanced players, however, may prefer to limit this move to Kings only (as it is in Klondike).
Eight Off is similar to Baker's Game, named after the mathematician C.L. Baker and a precursor to the more popular FreeCell. It is not as popular as its offspring, but is included in most large computer card-game suites, such as SolSuite, BVS Solitaire Collection and Eric's Ultimate Solitaire.