The most abundant and least regulated category on this page, which cuts both ways. A well-run retreat with a small group and an experienced teacher does most of what people want from a week away: routine, quiet, movement, no alcohol, other people. A badly run one is a hotel with a timetable.
The split worth understanding is between ashrams and retreat centres. An ashram gives you a fixed daily schedule you join, often starting before dawn, usually with chanting and karma yoga (unpaid work) built in, and it is cheap. A retreat centre gives you a programme you have booked, better food, a private room and a bill three to ten times larger.
Neither is better. They fail differently. Ashrams fail by being rigid, occasionally doctrinaire, and having had their own share of guru scandals. Retreat centres fail by being expensive and empty.
For people who arrive here because a psychedelic option is closed to them, the useful thing about a yoga retreat is not the asana. It is a week of structure, sleep, no drinking and no phone with other people around, which is a large part of what makes any retreat work.
A representative shape drawn from how these are typically run. Use it as something to ask against, not as any one centre's timetable.