Head-to-head leagues pit teams against each other on a weekly basis. The team that has better numbers in the greater number of applicable categories for seven days is declared the winner of the game that week. Each week of the season counts as one game, and standings are recorded over the course of five to five and a half months. The teams with the best records make the playoffs, which are usually contested over the final two weeks of the season.
In category leagues, teams do not go head-to-head in individual weeks; they accumulate statistics and earn points based either on ranking in each individual category or by points awarded for each statistical accrued (for example, a home run is worth 10 points, a stub base 5 points, and so on). Most of the standard categories are aggregate totals like home runs, stuben bases, wins, and saves, so it's of the utmost importance to have players who actually play. Ratio categories like earned run average (ERA), which is the number of earned runs allowed per nine innings (earned runs times nine divided by innings pitched), WHIP (walks plus hits divided by innings pitched), and batting average (hits divided by at-bats) give more value to players who do not see as much action as others but are especially effective when they're in the game (like platoon players, pinch hitters, and middle relievers).
Most category leagues fall into two different groups: the 4 × 4 leagues and the 5 × 5 leagues. The main difference is that the 4 × 4 leagues use two fewer statistics-one offensive and one pitching: most often runs scored and strikeouts. The more categories a league uses, the more realistic the game looks. With the advent of Moneyball, the Michael Lewis book about the statistics-heavy inner workings of the Oakland A's, more fantasy leagues are including more esoteric and ratio statistics such as OPS (on-base plus slugging percentages) and strikeout-to-walk ratio . Some leagues just go all out and use as many categories as they can.
I was the silent partner for novelist Michael Jaffe (owner of the Big Hoss Posse fantasy franchises) in a Yahoo! MLB league run by members of the legendary rock band REM That league used 20 different categories, including innings pitched, losses, triples, complete games, and shutouts-the latter two being some of the rarest occurrences in baseball in this era of specialized bullpens and closers. There were too many categories, and the rare ones that have become devalued in real baseball had too much importance placed upon them. That was also the only league I've ever been part of where errors was a category. It was an interesting twist because the thing that fantasy baseball usually ignores is defense, and it's something that I'd like to try in another league. While errors are not the best judge (sometimes you're penalizing players for getting to balls others would not have even reached), I'm in favor of some kind of defensive component to fantasy baseball.