When Americans think of Zen, they think of koans- those riddle-like questions that are supposed to lead you to enlightenment. Rinzai Zen has a structured curriculum of koan practice, so you work through the koans in a fixed sequence and get a “Zen diploma” when you finish them all. Soto Zen did make use of koans at an earlier stage in its history, but it no longer does. So what does Soto Zen practice involve, if not koan study?
In Soto Zen, you just sit. Silent, seated meditation is the core of Soto, with the goal being a mental state of completely awake and alert clarity without allowing the mind to fixate on any objects of thought. If that sounds easy to you, give it a try- it's an extraordinarily difficult mental state to achieve and maintain.
The differences in practice between Soto and Rinzai are also reflected in their attitudes to Kensho or “enlightenment.” Rinzai practitioners tend to think of Kensho as a huge, life-changing experience, while Soto practitioners tend to see the practice of correct meditation as being enlightenment in and of itself. Many Soto Zen Buddhists practice seated meditation for years and years without ever having the big “enlightenment experience” familiar from so many Zen anecdotes, yet this is not generally seen as a problem. In Soto Zen, sitting quietly and paying attention is both the means and the end.