Seated meditation facing a wall, in a room, on a schedule, with a form governing how you walk, eat and bow. Physically stricter than an insight retreat and less psychologically explanatory. Nobody will tell you what you are supposed to be experiencing, which is the point and also what people find hardest.
A sesshin is not an introduction. It is several days of zazen in periods of roughly forty minutes, separated by walking meditation, with formal meals eaten from nested bowls at your seat. The form is exacting and learning it is part of the practice rather than an obstacle to it.
The Western Zen centres worth knowing are monasteries that also take guests. That means you can usually arrive as a residential guest or short-stay work-practice student first, sit the ordinary daily schedule, and find out whether the form suits you before committing to a sesshin.
Zen is the tradition least interested in the states this site otherwise catalogues. If you want an experience, this will frustrate you. If you want a structure that holds up over years, it is the most robust thing here.
A representative shape drawn from how these are typically run. Use it as something to ask against, not as any one centre's timetable.