What are electrolytic caps?

I have a pair of Spica TC-50s (circa 1989). I was reading an interview with John Bau (inventor of the Spica speakers). He says in the interview that the "electrolytic caps go south after about a decade."

1. What are electrolytic caps?

2. What does he mean they "go south after a decade"? Does sound quality significantly decrease?

3. Are the caps something I can easily replace and how? Sounds like you need to get a matched pair with tight specs.

Thanks for your help! The interview link and quote are below.

John

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http://www.enjoythemusic.com/magazine/equipment/0900/spicafever.htm (quoted section below)

Quote from interview:

JB: For TC-50 and Angelus; replacement of the electrolytic caps with film types will help (they start going south after a decade or so anyway). Exact design values should be used (not the values printed on the parts; we batched all our components to better than 0.1% groups, and paralleled them to achieve exact values). I'd have to dig up the schematics to find the target values.