What is inside an exhaust system?

[Lowejackson@domain.elided]

This may seem a little strange, but what is inside an exhaust system?  The 
reason I ask is after driving home from work I noticed some long fibres 
hanging out the rear exhaust pipe.  They were long strands a few feet in 
length.  I am not sure if someone put them inside the exhaust or I collected 
the contents of the exhaust.

If someone had stuffed this stuff inside, could there be any potential damage 
caused by having a partially blocked system?

Steven
164 TS (1993)
Cumbria UK

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