Question for Dwellar musicians

[squirell nutkin]

Here's something I just don't get and I wonder if the collective dwellar brain can 'splain me:

In music people talk about things being 3/4 time or 4/4 time etc.

I've looked at wikipedia and had people tell me it's the number of beats per measure.

Great, but the word time throws me off as does measure. Is there a set length of time, i.e. seconds that a measure lasts? if so, why not say it is 4 beats per second? or can 4/4 time be played fast or slow? How can it help you if 4/4 time can be fast or slow, then it would be sort of meaningless, I imagine to have a time signature at all.

from wiki: "In musical notation, a bar (or measure) is a segment of time defined as a given number of beats of a given duration"

OK, so what is the duration? how can a beat be drawn out? I think of a beat as being the musical equivalent of a point in space only slightly chubbier.

I understand that 8/4 time is faster than 3/4 time but I don't get where the actual time is.