What is the difference between 1st and 2nd class postage?

I want to send a letter to germany and it costs me 87p (2nd class) today I wanted to buy stamps and I could only buy 1st class stamps for 60p.

So my question is: why is 1st class cheaper than 2nd class? And can I use two 1st class stamps to send my letter or do I need one 2nd class stamp?
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