No other component makes more of a difference to your on-bike comfort on your bike than your saddle. Get it right and you can ride all day, day after day. Get it wrong and you're in pain within minutes, and it doesn't get any better.
The fit of a saddle is determined by the shape of the hull, and its width. How wide a saddle you need depends on the spacing of your sit-bones and how upright you sit, while the right saddle shape is determined by the details of your shape. Some people get on best with flat saddles, some with dipped shapes, and so on.
Because riding a bike puts pressure on the soft tissues of the genitals, women usually need different saddles from men, shaped to reduce the load on sensitive areas.