why should I use `&amp;` instead of `&` when writing HTML for my site?

Where can I find a list of other symbols that I should be encoding? (the bar / too, right?)

What problems could I have if I paste the symbol right as it is into the html?

**The thing is**, I have a few affiliate links and I'm worried that, if I write them with the & in some cases, for some reasons, because of the specific browser or device... that the info in the url wouldn't be passed correctly. 

Hopefully someone can clear my mind? 

Thanks!