For many generations it seems that women wore flowing garments, dresses, or skirts, whilst only men wore trousers. It is probable that the industrial revolution had something to do with women taking to wearing trousers as being more suitable for daily manual work, and then, gradually the rise of feminism may also have contributed to the idea that women could wear trousers too.
Fashion designers have developed the theme of trousers for women in various ways. There were neatly cut trouser suits worn by some film stars like Marlene Dietrich, but in the main pants for women were associated mainly with leisure wear, ideal for riding, gardening, or the golf course. When mens' denim jeans became not just work wear, but fashionable, women soon followed suit with designs that would make the most of their anatomy. Few would deny that a woman in a pair of tight fitting Diesel jeans can look as sexy as one in an evening dress.
These days jeans are acceptable almost everywhere. There are probably just a handful of clubs or nightspots that would deny a lady entry because she was wearing jeans, such is the almost universal acceptance of this mode of dress for women, from casual to formal. Some women would even apparently wear jeans to a wedding, presumably dressed up somewhat with an elaborate top and high-heeled shoes.
Fashions come and go with jeans but a good pair should fit well, be slightly below the waist, and slightly tapered or cut to the boots. That, at least, is one designer's opinion but there is a current trend for high-waisted jeans, especially for the older woman, and there have been numerous other styles that are still out there and OK. There is the Classic or regular fit, the Bootcut with slight flare at the leg opening, Easy or relaxed fit with low-rise waist and plenty of room, Slim, and Skinny with a snug fit around the bottom. There are also of course different colours and textures as well as styles, with blasted or corded as a couple of examples.
There are plenty of women who believe that women's jeans are better designed than mens and some even persuade their partners to wear them! Jeans will continue to be cool for some time to come.