You've seen this type of thing before. Given a category, name lots of things which fit that category in a limited amount of time. Don't doze off yet, though, because A to Z is fresh enough and good enough to add to your party game stack.
Each player (or team, if more than four people play) gets their own plastic scoreboard with a 5x5 grid of circular depressions, each labelled with a different letter of the alphabet (X and Y cohabitate). Two dice are rolled on each turn. The first dictates the time limit (15 or 30 seconds), the second indicates which of the six categories on a card will be used. The timer (a noisy wind-up, flips-over-automatically-when-done type) is begun, and the current player starts rattling off as many things as he can which fit the category.
As each item is named, its first letter is covered with a transparent red chip on the player's scoreboard. The first player to cover the entire board wins the game.
Instead of rolling a time limit, the first die might come up with two other results. The first skips your turn, but lets you cover up any one letter of your choice instead— handy for Q and Z. With the other option you still get a category and talk for 15 seconds, but instead of covering your own board you pick an opponent and remove chips from theirs. Choose that opponent wisely— you can be sure, what comes around goes around.
There are two things which make A to Z succeed. The first is that each round is very short— no more than 30 seconds— so you're not waiting around long for your own turn. And you can play along silently with opponents, gloating over items you thought of which he missed.
The second factor in this game's success is its selection of categories. These are, by far, the best categories I've seen in a game of this type. We've all played many category games, and they all seem to have the same tired and dull categories. A to Z has some of those, but because each category is used in such a brief time-frame, there are many unusual and, often, limited categories to spice things up. Samples include "TV game shows", "Star Trek characters", and "Breeds of cows". Admittedly, that last one wasn't too popular. Broader categories include "Countries south of the Equator", "Musical terms", and "Things found in a doctor's office." It's a balanced and lively set, neither too hard nor easy.
Strategies change as the game progresses. Early in the game players have empty boards and just rattle off everything they can think of to try to fill as much as possible. Later, uncovered letters beckon and players feel satisfied if they can cover just a couple of them.
A to Z is nicely produced, offers a new twist on the tired category theme, and can be played by the whole family. Sometimes, one more trip to an old well can pay off.