Lightroom has two types of Collections - 'Dumb' Collections and 'Smart' Collections.
Well, I wouldn't call non-Smart Collections "dumb" I would call them "simple". A simple collection is just that, a collection of images from various folders of you catalog brought together manually...you drag and drop them. Also note you can create a Collection Set to further you simple collections.
Smart Collections are correctly described as being the results of user selected criteria (it's not wrong to call them a saved search criteria but I think it's an oversimplification).
In a SC, you decide what criterial will be applied to the results and LR automatically updates the results. If you have a SC for "Reds" (which Mike does a lot and actually ships in LR) then the moment (or in the background, it may take a few moments to update) that newly labeled Red image will show up in the Reds SC.
Simple collection need to be manually updated and changed, Smart Collection are updated automatically by LR. (and if you want to think if "simple" as "dumb" that's ok but not really PC).