[sdiy] which caps?

[Roy J. Tellason]

I've been scrapping some stuff lately (mostly VCRs in the past week or so),  
and am wondering about what drives the choices of some components,  
particularly capacitors.

Obviously if you're looking for really large values (in excess of 100nF) then 
the only sensible choice is electrolytic.  And for really small values the 
choice appears to be ceramic.  But in between?

On one board I found lots of electrolytics,  ceramic disc,  polystyrene (?), 
and mylar (?),  and the occasional tantalum.

What would suggest the use of a tantalum over the usual aluminum electrolytic?

What would suggest the use of an electrolytic when you're looking at a value 
of say 100nF?  I've also got mylar and ceramic disc in this value.

Obviously costs are one consideration (though not so much for me as for a mfr. 
since I have this massive pile of scrounged parts :-),  but I'm also 
wondering what makes this choice go one way or the other for the simple 
fiddling around I'm into doing here...

I know one time years ago when Radio Shack first started stocking the 567 
chip,  I got a hold of one and did some playing around with it.  Built a 
circuit,  and a 555 to drive it with,  and it just didn't work.  Pulled out 
all of the ceramic disc caps I had in there and put mylar (?) ones in there 
instead and it worked just fine.

Can anybody help shed some light on this?