A
microgame is a loose term that describes a certian type of
game. Idealy it should have some, if not all, of the following properties.
Small - Micro in fact when compared to the bookshelf games put out by companies like Avalon Hill and SPI. The maps, game pieces and rules should be able to be stored in a container that could fit inside a jacket pocket. Many of the early microgames came in zip lock baggies.
Inexpensive - In other words cheap, but not lacking in good materials or playability. The first microgames cost about $2.95 and got up to $10 at their height. Today they could go as high as $15.
Quick Playing - While bookshelf games can take days to learn and weeks to play, microgames often can be learned in under an hour and played during a lunch break.
Conflict Based - This is perhaps the loosest part of the microgame definition. Many people say only war or conflict games fit here, others are open to RPG style games while still others will allow games like Pass The Pigs and CCGs (Collectable Card Games). Personally I draw the line at CCGs and pure luck (ie dice throws with no real strategy).
Some of the better known microgame publishers are/were Metagaming Concepts, Task Force Games, Steve Jackson Games, Yaquinto, Dwarfstar and GDW to name a few.
Far from a thing of the past microgames are still around with us today. DTP games, PBeM tools like Cyberboard, and great game design keep this an every growing interest.