Population and Unit Limits are simply numbers that sit on on your screen and show your economy's current capacity. You can use them as an indicator of just how big and scary your forces have become. The Population Limit sets an absolute cap of how much you can ever build at any time; the Unit Limit shows you the maximum you can currently build at that moment. Unit limits can never exceed Population Limits. If you try to exceed your Population Limit you simply won't be able to generate any more units.
In order to increase your Unit Limit you have to start constructing special buildings to "develop" your economy. The usual RTS convention make these structures look like farms or houses, or perhaps a warehouse of some kind. These structures are the imaginary "housing" and supply depots needed to keep your forces rested and topped up - not that you ever see such activities in action: most games skip all that boring stuff in the same way Anime films skip animating dialogue scenes and focus lovingly on the fire and brimstone instead. The more houses you build, the higher your unit limit goes. e.g. in Age of Empires, building a House allows you to support five units; so to build a force of fifty guys, you must have ten Houses. It doesn't matter whether its five mighty galleons, five villagers or five hamsters, a House grants you license to build five additional units.
In StarCraft or Cataclysm, its a little different: instead of housing they follow a warehousing mentality. Some units consume more support and materials than others. Creating a StarCraft Marine uses one point; a Siege Tank uses two, while a giant Battle Cruiser takes six! So despite StarCraft's Population Limit of 200, its rare that you ever build that many units. Building six Battle Cruisers needs the same storage capacity as thirty-six Marines. The more big units you create, the more warehouses you have to build just to support them. Its a rather neat way of making sure everyone can't just bludgeon each other with masses of big units and also helps balance the game a bit more. After all, effective strategy is as much about managing and expanding your economy as it is about thumping the opposition.*
Once built, the support building is inert. It just... well, it just sits there and takes up precious real estate. You won't really notice it ever again until the enemy flattens a line of them. Suddenly your Unit Limit drops below your current force's support level and you've got a nasty situation where the enemy is attacking and you can't build any fresh reinforcements. Destroying support buildings is a very effective way of hamstringing the enemy, but it can take time to destroy so many buildings for a relatively minor advantage. You might find that they are lightly defended or being used as makeshift walls. If you do demolish your enemy's unit limit they will be forced to spend time and resources rebuilding their housing first before they can replace their casualties, giving you plenty of time to wreak merry carnage on them.
In recent years, there has been a trend for Population Limits in games to get smaller, despite quantum leaps in processing and graphics power of PC's. Games would usually have caps of about 150 to 200; now its not unusual to see titles like WarCraft III (unpatched) drop down to 70. It almost goes without saying that a game with a low population limit is one that has a high degree of unit micromanagement. The less units you have, the more attention you can spend on individuals. Games with high Population Limits tend to be more big picture; more crowd driven. Units are more impersonal and more expendable.
Last reformatted Sat, Apr 30 2011 by Lindsay Fleay
* If you want to play a game where it really IS only about superior numbers, I can recommend any Westwood Studios strategy title: Command & Conquer, Red Alert, Emperor Dune, etc. I just bought Emperor: Battle for Dune recently, feeling that RTSC had been neglecting the Westwood titles. Now I remember why.