This may help: Trousers - Wikipedia, the free encyclopedia
In the US, we tend to wear "pants" instead of "trousers" (though we certainly understand the word).
In the UK, "pants" are what we in the US call "underpants" or "underwear." - Panties, boxers, briefs, etc.
I think of "slacks" as dressier pants. I might say I need a new pair of slacks that are suitable for work.
Jeans are a type of pants, but not a type of "slacks."
Again - my words are US, not UK.