The Mechanics of Stronger/Weaker Currencies

[millermandil]

Hi There. Could someone please explain to me the mechanics of 'one currency being stronger/weaker compared to another currency/ other currencies' and what impact does this have on the stock market internationally? Im learning about the US Commodities market, so if possible, specifically in relation to the latter.

Furthermore, how does a currency become weaker/stronger...GDP?, economic growth?, interest rates? Employment?...

Thanks.