How does an air filter work?

Before I ask a really stupid question (I know this one probably sounds pretty stupid)...can someone fill me in on the workings of the air filter and housing ....ie air flow direction, holes etc. I'm replacing some of the the vacuum hoses and on my 90 XJ and something just doesn't seem right.

In the XJ the air comes in the front of the air box, flows up thru the filter and down that thick hose to the throttle body, that little hose keeps crankcase pressure to a minimum and burns any loose hydrocarbons in the engine.
Some filters work better than others, you want a filter that filters out the smallest particles and still supplies enough air for the engine to work.
I use either purolator or frams in my XJ and Fram in the TJ, the TJ fram has a foam prefitler that I like and wish the XJ had it. I'm considering putting a TJ airbox in the XJ so I can standardize to one filter size.

Perfect thanks very much. That's basically what I thought, although for some reason I was totally blind to the front intake hole and couldn't figure out how air got into it (I'll blame it on poor lighting :speepin: )....there is a bolt missing from the bottom of the airbox so there's a little hole down there (I wasn't sure it was a missing bolt but knew the hole was too small for any sort of real intake....but your description makes it obvious). I got a fram filter...cheaper than I thought it would be (about 8 bucks)
Did a total coolant flush and changed the thermostat yesterday so I thought today I'd get the oil changed and change that air filter. Thanks again.