There are 3 different categories of settings which are voted on by players at the start of a game. Game strategies will vary completely by what the game settings are. For example, Squad Mode limits the amount of units out on the map at any one time but does not limit the amount of heroes so the result is that players use heroes much more often in squad mode than in any other game setting. Currently the most popular game mode is "Classic Marine Arena" while the most popular game speed is "Fast".
Classic Marine Arena
Squad Mode - Unit caps of 100 marines and 50 mercenaries for each player
Classic Marine Arena - 0 minerals at start.
Fast - 500 minerals at start. Double income at every 10 minute mark.
Faster - 500 minerals at start. Double income at every 10 minute mark. No Saviors.
Fastest - 500 minerals at start. Double income at every 10 minute mark. No Saviors / No base defenses.
Yes - At the 1 hour mark, sudden death becomes available which destroys every unit on the map (including heroes) / removes all base defenses and saviors. The team holding the map center gains 50 minerals each every 20 seconds
No - Game continues as normal