hey i didn't mean to offend you. i was only joking. we really do support any type of responsable fish keeping, which you are obviously doing seeing as your asking questions. don't take those comments as how MFK or even i see this forum, i was just trying to lighten the atmosphere.
so back on topic, kinda. i'll give you my understanding of sumps, hope it will help rather than confuse.
a sump is basically just a separate container to hold extra water and usually utilized to setup/hide filtration. usage consists of a way to transport water from the main tank, commonly an overflow, to the sump and then a pump to the return the water to the tank.
filtration in the sump can range from relatively simple to complex, dependent on the needs and experience of the hobbyist. if being used for filtration the sump is separated into different compartments using baffles to set water levels.
although i do not speak for everyone(and please don't judge this site solely by me as it is a wonderful site where you can learn much) the simplest and one of the most effective is the w/d(wet/dry or trickle) filter. the water is spread across a bio media

headbang2scrubbies

) via a drip plate, spray bar or both, which is only partially submerged in water(heard everywhere between 1/3-2/3). water should be mechanically filtered before the w/d, easily accomplished by placing polyfiber on the drip plate. after the water passes through the w/d it's off to the pump and back into the cycle.
hope this helps a little. a sump would probably be great for a terrarium since there is only a little water available in the main tank(usually) having the sump will increase capacity and help stabalize water quality.
here's a quick paint pic to maybe make my rambling understandable(please excuse the horrible art

)

bio media in pic is scrubbies

(you should search the forum for this term to understand the love

)
finally here's a link to start with, always gives me inspiration to try new filtration ideas.
http://www.monsterfishkeepers.com/forums/showthread.php?t=97647&highlight=filtration+pic+thread