Famously the Sims and similar games have been described by some designers as Toys and not "really" games.

I'm curious if there is a good answer to what makes something a game.

For example many companies sell Sudoku games - EA has an iPhone one, IronSudoku offers a great web based one, and there are countless others on most platforms.

Many newspapers publish Sudoku puzzles in their print editions and often online.

What differentiates a game from a puzzle? (or are all Sudoku "games" misnamed?)

I'm not convinced there is a simple or easy answer - but I'd love to be proven wrong. I've seen some definitions and emphasize "rules" as core to something being a game (vs. "real life") but puzzles have rules as well - as do many other things.

I'm open to answers that either focus only on computer games (on any platform) or which expand to include games and gameplay across many platforms. Here to I'm not fully convinced the lines are clear - is a "game" of D&D played over a virtual tabletop with computer dice rollers, video & audio chat a computer game or something else? (I'd lean towards something else - but where do you draw that line?)