Before I get in too deep with the jargon of pants it’s important to point out how fluid these terms are. Despite what we might read here or anywhere else, expect to see some of these terms used interchangeably elsewhere. I think of pants as the catch-all term, while slacks tends to include trousers, chinos, and khakis. Of course parts of the UK say pants instead of underwear and for some people, slacks are only for women.
Trousers
Trousers are your nice pants. They can either be paired with a suit or sold alone. Modern trousers were made fashionable by Beau Brummell, the original Dandy, in the early 19th century. Brummell believed, and most writers today agree, that trousers look best when they’re tailored.
http://www.realmenrealstyle.com/mens-trousers-style-fit-fabric/
Chinos
This term originally referred to a type of fabric designed for military use in the 19th century. Today they are semi-casual pants typically made from a cotton/synthetic blend of fabrics. This makes the chino lighter and more elastic than some other pants. Generally, chinos will have their stitching hidden, while khakis will have visible stitching along the seams.
http://www.stylesamplemag.com/mans-guide-to-chinos-what-are-chinos-how-to-wear-them/
Khakis
Another type of pants with military origins, khakis originally referred to both the fabric and the color. Khakis today are 100% cotton, have visible stitching, and are more casual than chinos. Think of khaki’s like a slightly dressier alternative to jeans.
THE DETAILS ON KHAKIS | STYLE & FIT – WHAT EVERY GUY SHOULD KNOW
Cargos
While now fashionable, cargo pants have their origins in the military and the working class. The distinguishing feature of cargo pants are their pockets and their durability. They were made to endure the rigors of manual labor. Avoid wearing cargo pants to the office but don’t be afraid to dress them up.
http://www.ebay.com/gds/Mens-Cargo-Pants-Buying-Guide-/10000000177631168/g.html
Cords
Corduroy is a fabric distinguished by its raised ridges of velvet. The ridges are called wales and they can either be small and tightly packed or thicker with more space between them. While corduroy pants tend to have smaller wales, it all comes down to style and comfort. Do they look good? Do they feel good? They are decidedly casual, but still acceptable for a Friday at the office. Not for summer wear.
http://www.mademan.com/mm/how-wear-corduroy-pants.html
Leather
These pants tend to either look awesome or ridiculous. Made famous by musicians and celebrities, leather pants take a bit more work to pull off than our typical pair of pants. They can also be more expensive so we should make sure we buy a pair that fits and can be used in lots of outfits. Remember, leather is hot so save these pants for the fall and winter months.
http://www.livestrong.com/article/1005286-fashion-tips-guys-wearing-leather-pants/
Jeans
Perhaps the most universal type of pants in the US, jeans as we know them have been a part of American culture since the 19th century when Jacob Davis invented them for mine and ranch workers. Today they retain their use as a common work pant in addition to being suitable for both casual and fashion wear.
http://en.wikipedia.org/wiki/Jeans
I did a lot of reading before putting this one out there. Here are a ton of references:
http://www.ebay.com/gds/The-Complete-Mens-Casual-Pants-Buying-Guide-/10000000177631187/g.html
http://www.atailoredsuit.com/mens-trousers.html
http://www.orvis.com/news/products-we-love/chino-vs-khaki-do-you-know-the-difference/
http://www.diffen.com/difference/Chinos_vs_Khakis
http://en.wikipedia.org/wiki/Chino_cloth
http://en.wikipedia.org/wiki/Khaki
http://www.realmenrealstyle.com/mens-trousers-style-fit-fabric/
http://artofstyle.hucklebury.com/pants-men-need-know/
http://uk.askmen.com/fashion/fashiontip_150/165_fashion_advice.html
http://en.wikipedia.org/wiki/Trousers
http://english.stackexchange.com/questions/79710/difference-between-slacks-pants-and-trousers